Here's is the first part of a handy glossary of useful terms. It is meant for the metal initiate but even if you've decades of metal music behind you, you could do worse than fast-reading through. If you'd like to mention other confusing terms relevant to metal, go ahead and I'll be sure to address them in the second part.
A riff is the foundation of metal music, but it's not its invention. It's usually just a short melodic or rhythmic motif as found in nearly all popular music. What makes it a riff and not just one of many themes musicians might employ in the course of a song is that it is incessantly repeated, both many times in a row and then returned to later in the structure. Its purpose is to be catchy, hook the listener. In pop music, the return to the main theme of a song is usually through harmonic progression and other means so as to be a climactic event. Often in metal music is it instead obfuscated by a great number of additional riffs of equal strength. This is called a riff salad type of composition. The term is pejorative generally, though there are many bands that achieve peculiarly successful results by feeling their way through a composition with endless riffs.
Riffs are usually guitar phrases in metal, but they do not differ fundamentally from vocal or keyboard hooks in other pop music. Metalheads like to draw a sharp distinction between their stuff and pop, usually by mentioning the surface similarities between the former and classical music. In truth, the vast majority of metal is usually less or equally composed to pop music (and certainly less composed than any classical music piece) because it is too strongly anchored to the middle-structure of the riff. Micro-structure (nuance and change within the phrases) and the higher structure (the flow of the whole composition) are usually neglected or left to occur completely reflexively.
The definitive statement on the progression of mainstream metal from the '70s onwards is that the riffs got plentiful and more complicated as the genre grew and harmonic sensibility (once inherited from both folk music and Beatlesque rock n' roll) took the back seat. Metal songs from the '90s may have more riffs than whole albums from the '70s. Yet this doesn't mean the riffs cohere to an overall composition with flow and movement. Exactly because metal focuses on the riff structure, tonal shifts are neglected in metal, as is supporting harmony. Most metal songs either keep to a single key throughout or are all over the place in a chaotic flurry of riffs upon riffs that move around the neck seemingly at the whim of the performers hands. The usual harmonic support of most metal riffs is the power chord.
The Power chord
A simple two or three finger chord on the guitar where there is a root note, its fifth and optionally the octave. This is the foundational sound of metal (and punk) rhythm guitar. Often riffs are made out of melodies that are then played with power chords in parallel, that is where the fifth in the chord is never diminished or augmented to fit the scale the music is in. For the metal musician "fifths always sound right". It's useful to keep in mind that metal music, no matter how ambitious it is in scope, is predominantly made by musically semi-literate artists. To understand the compulsion of riffing, one needs to just pick up a guitar and play the riff of 'Smoke on the Water'. Addictive, isn't it? Metal music is the result of masses of self-taught and/or music-theory deprived musicians toiling in private to perfect the riffs that come to them through improvisation around short forms. This explains why harmonic movement is limited (because harmony needs more rigorous study to be understood). However, this is also one of metal music's greatest strengths because through the unorthodox obsession with the riff, musicians arrive to novel solutions to compositional issues. The end result might not have the usual grace of thoroughly composed music, but it has its own quirky beauty for what it is. Constant Power chords following the melody destabilize it by not providing a colorful chord progression that foreshadows resolution. Indeed, often metal melodies resolve in unorthodox ways, or do not resolve at all. This, along with the constant fifths makes them sound equally forceful and driving at all the points of them, yet slightly confused. Odd, but full of character. Dynamic nuance is traded in for force. To convey accent and direction to the melody, power chords are often alternated with palm muted single picked notes, usually on the top two strings.
The practice of resting the cut of one's hand on the guitar bridge, partly muting the strings and then striking them. The muffled sound sacrifices tonal clarity for punch. Usually employed as a rhythmic fill-in between nodes of the melody, though some bands like to compose in that space as well. The playing of palm muted triplets is very characteristic of metal. Introduced by Black Sabbath and endorsed heavily by Iron Maiden, often called the 'gallop' riff, like in Rossini's William Tell overture. Or to make a more telling reference, the Lone Ranger opening theme song. A mid-'80s development on palm muted rhythm guitar playing occurred with the rise to prominence of American thrash metal. Since then, palm mute chops are rated highly in the metal world and we've come to the point where no matter how distanced the sub-genre a mainstream band belongs to is from thrash metal, it's a given that they'll have post-thrash technical rhythm guitar chops in there somewhere.
The solo & the lead
The practice of one of the instrumentalists taking anything from one to sixteen or more bars of time to perform virtuso feats on their instrument. In metal music it is most often the guitarists who take solos, unlike in jazz where usually the whole ensemble takes turns improvising before returning to the theme. Unlike jazz again, metal solos are usually composed beforehand, perhaps with some interpretative space somewhere in the middle. Sometimes metal solos are harmonized between two or more guitars. Again, the trend from the inception of the genre towards today has always been on showcasing increasing technique and speed. Guitar solos in metal music sometimes offer more compositional interest than the song they belong to but sadly, it's more often the case that, characterized by the frustration of the self-taught metal musician, once the time for a solo is up, almost anything goes even at the detriment of the mood of the rest of the composition. Metalheads easily accept intrusive solos in their music and celebrate their highest virtuosos with a sense of vicarious pride.
The lead is a more nebulous term. I tend to use it to describe distinctive guitar melodies that occur in metal music and do not belong structurally, to a solo bit. Often it's used interchangeably with the term solo. Many atmospheric metal bands employ almost constant dirgeful lead guitar that wouldn't be done justice to if they were called solos. The distinctions are blurred once the leads become technical and busy enough to sound like the typical metal solo, or when lead guitar work bridges into the solo structure proper.
Taking an instrument tone and running it through an amplifier, overdriving it so much that the waveform peaks and rumbles in an aggressive manner. It is extremely characteristic of metal that it took what was once considered an unwelcome byproduct of amplification, and made its most enduring banner. Something ugly into something beautiful.
Guitar tone in metal is over-distorted, that is to say its dynamic range is flattened. Metal guitars are often schizophrenic: either very loud or completely silent. Metal guitar playing technique rests on this default that less manual force is required to play an overdriven instrument than a clean toned one. Many would be surprised on how softly modern metal guitarists play if they could hear their pre-distortion clean channel. Metal music compensates for the lack of dynamic range of its distorted lead instruments by applying distinctive sound design principles. Guitars are overdubbed extensively and panned around to create a space that the original rehearsal room in which the performance was captured, is very unlike. Guitar solos/leads are overlaid in higher or lower volume than the guitar initially outputted. The end result is strangely Baroque: dynamics are more a matter of introducing or removing extra voices and/or making the existent voices busier, than they are the result of the performers hitting their instruments softer or louder. As you might have come to expect, the trend from the '70s to today is to make guitar distortion more robust, equalized and over-compressed. Modern metal bands often sound inhuman and sterile.
One of the clearest signs that you're listening to metal music is the metronome-like clicky-clicky sound in the background (or in some cases, right in the foreground). That's the drummer kicking one bass drum with each leg, or the same bass drum with a double-pedal, at sixteenths over tempos going as high as 230 beats per minute. Much is made out of a metal drummer's athletic ability to maintain constant battery of this type for long durations and at high tempo. In extreme metal in particular, this skill is valued over traditional strengths of drummers like solid meter and creative feel.
The reality of it is that often the end result is much unlike the sound captured in the rehearsal room. The natural sound of the kick drum is much flatter than what ends up on the mastering tape, where equalizing and compressing of the signal is employed to make it punchy and clicky enough for the constant sixteenths to register as they do. Often the kick serves only as a trigger for a sound of a digitally treated drum sample to come in and replace in the final mix. Finally, uneven double-bass triggered kicks are easily quantified in the mixing stage to become metronome-perfect. The trend of metal music from its inception onwards is to remove human blemishes on recording takes by any means necessary. The attraction to this sort of battery is the masculinity of linearity: Machine-gun like, rigid and martial, double-bass anchors the rhythmics of metal and propels the music in a single direction. Doubled with metal post-thrash rhythm guitar, their thundering unison is the sound of modern metal.
This is like, extreme, dude!
Along with 'brutality', one of the terms metalheads enjoy employing the most when discussing their favorite music. Extremity is a culturally flexible label. Metal music was once considered extreme on the whole as compared to blues and hippie rock, as they were once considered extreme compared to what came before them. Many of the early incarnations of metal are now considered tame enough to be placed next to other 'classic rock' acts that dads might listen to.
Adverse to this fossilization, aficionados of extremity are always looking for the next band that will play faster or slower, harsher or meaner, more sterile or sloppier than everyone else, yet still retain musically discernible structure. Extreme music as a concept has almost eclipsed metal music. It's one of many genres such as drone, ambient, metal-core (a combination of metal sonics and hardcore punk aesthetics/ethics) or industrial, under the same banner. Listeners interested in metal are expected, in a modern cultural climate, to be interested in all these other cultural artifacts as well.
In every case, extremity is judged by how expertly an individual band hones on one of the aspects of their sound design. The fastest or heaviest or slowest band wins. The effect of this is that many of the so-called extreme bands are anything but. Because their scope is so narrow, the resulting music becomes normalized, boring, inoffensive. Furthermore, it seems we might have reached the limits of sound. How much faster or tighter can a band perform before the result crumbles in incoherency? Will 10 miliseconds make the difference between last year's most extreme band and the next years?
A very strict method of composition where many melodic voices are overlaid at the same time, devised by J.S. Bach. The particular composer is loved by many for the natural beauty of his work and the almost mathematical depth to his process.
In metalhead terms, the much less structured attempt at overlaying three or more independent leads at the same time. The difference is one of rigor. It is very telling however, that untrained musicians such as metalheads often indulge in this sort of composition in a trial-and-error way. Composing every little bit by asking 'does it sound good? If so, keep it, if not, move the fingers around on the fretboard a little'. What could it be driving the ill equipped towards such grandeur?
A word many extreme music record reviewers love to throw around. When two notes are struck at the same time, there is a melodic space between them that characterizes their relationship. Hitting a piano key that we consider the root note, and the key right next to it, is a second interval, for example. The power chord is made of the root note, the fifth interval and the eighth interval, as mentioned.
Some intervals carry inherent connotations for us. The minor third is often tragic, the major third hopeful. The fifth sounds, well... perfect. There are evolutionary theories on why we have such strong primary reactions to frequencies but for the purpose of understanding dissonance, just keep in mind that specific intervals sound inherently uncomfortable to most listeners. The minor second interval is the most distinctive example. Metal bands that employ these intervals (and stack them, that is to say, play many such intervals together at once) are said to be "caustic, dissonant". This effect is considered pleasant in its unpleasantness for many. The theme of masochism in metal music listening is one we will be returning to often, as it ties in with the romantic ideal of pain as purpose.
A word not many extreme music record reviewers seem to throw around.