The telltale sign that you're listening to not just metal but extreme metal in particular, is a certain busy drum rhythm that aficionados call the blastbeat. Metal adopted the technique from hardcore punk initially and not without some internal friction has it become a metal music staple. To understand the blastbeat one has to first understand the polka beat, and to understand the polka beat one has to know a few things about rock and roll in general.
Rock rhythm is usually in 4/4 at moderate pace, with accents on the first and third beat (kick and snare, respectively), divisors being hits on every count on the hi-hat. Go on, play that in your head. You're playing the backbone of sixty plus years of music right there. This is a very simple beat which allows for a lot of personal interpretation, variation and humanization. Many rock drummers enjoy playing slightly in front or behind the beat to suit the feel of the music. Feel, generally, is more applicable in slower tempos. The faster music gets, the more important it becomes for the divisions to occur clearly.
Early metal music added a lot of propulsion to the rock drum formula by adding syncopated kicks between the snare on the third beat, or straight up eighth notes on them throughout and other such slight alterations. However, the faster the rock beat became (and as noted before, metal music tends to the extremes) the more it becomes simplified to the 2/4 basic beat of polka music by necessity. To clarify, polka music and culture have nothing to do with metal music, it's just a common name for a certain technique: Kick - snare with no divisors between the two accents, just a hi-hat or other cymbal on the beat. This beat became predominant in metal during the heyday of thrash and somewhat earlier chronologically for punk music. The difference between the two schools of drumming is mostly one of rigor: metal drummers obsess over metronome efficiency at high speeds whereas hardcore punk drummers were fine with being sloppy if the end result connoted immediacy and panic. In either case, the polka beat has a very driving pace, perhaps physiologically it could be likened to running, one foot after the other with equal accent on both (drummer skill and sound design allowing).
Now, extreme music being what it is, the polka beat was eventually pushed to its extreme too, especially within the confines of hardcore punk, a type of music not very concerned with constant clarity like comparatively virtuoso metal music. The polka beat pushed until the distinction between the first and second accents is unclear, is a blastbeat. To the uninitiated, watching a drummer play a blastbeat seems as if he's hitting everything in unison on every accent, hence 'blasting'. However as the ears become accustomed to the flurry, the one-two division can still be felt. Blastbeats were introduced in metal music largely by Napalm Death and perhaps made a more distinctive impression in the work of one of the early death metal band, Morbid Angel. They were used by them usually in brief statements and embellishments. Being a hardcore punk staple, they were initially seen with some suspicion in the metal world, but in the end the need for speed won over the argument for debatable musical merit. Double-bass drumming and the capacity for an even blastbeat at high speeds are the bare perquisite skills for modern day metal drummers by result.
Blastbeats have become increasingly faster too, pushing speeds of 250 bpm (to get a grasp of that number, it's about four hits per second). Athletic and dexterous metal drummers now employ a host of specialized techniques to keep up at this pace, and those with such ability are highly sought after in the extreme metal world. This explains - to a degree - why drummers tend to belong to a plethora of bands and side-projects at the same time. I will not venture an extensive guess as to what other psychological and educational forces at are play that contribute to this, suffice to say that extreme metal drummers might enjoy putting most of the emphasis on the 'extreme' part of the equation: if I get to bash more stuff/faster, yes I will play for your band.
Blastbeats became common in two main genres of metal music: death metal and black metal. They are employed in these contrasting types of extreme music in subtly different ways. My opinion is that the biggest telling on whether you're listening to death metal or black metal is not aesthetics and lyrics and vocal styles, but the manner in which blastbeats are employed.
The blastbeat is a curious thing. It was borne out of the need for speed, as explained, yet it doesn't capture propulsion in the way its cousin, the polka beat does. Even that, in turn, lacks something a brisk rock beat has. Namely, divisors.
As mentioned, a rock beat has a suspended hi-hat hit on two and four of the 4/4 pulse. This serves as a dividing agent that tells the listener "this is one part of the linear progression of the rhythm, and this is the other". We thus learn to follow the kick and the snare as primary directional forces, they dictate a marching pace and purpose.
The polka beat, because it is faster and it becomes increasingly difficult to shove divisors in there, omits the hi-hat between the accents and only retains the hits that occur simultaneously with the kick and snare which are therefore, somewhat masked by their clamor. The result is a faster pulse but not necessarily faster pace. The polka beat in thrash metal instead depends on palm-muted rhythm guitar parts to act as divisors. Meticulously sound-designed to stand out in the mix, they dictate the in-between pace, and therefore thrash music sounds propulsive and linear as well.
However, the blastbeat, being as fast as it is, usually occurs in unison with the smallest element of guitar rhythm. And if it's not (there are some fast guitarists out there) the issue becomes one of clarity of sound design. And even if the sound is dry and crisp and properly mixed, then it is an issue of brain capacity: there's only so much processing the brain can make of disparate musical events before they all sound like a blurred soup. Extreme metal listeners have been training their brains for decades to keep up with that pace of information, but for new listeners, current top metal drummers might sound unimpressive in any other metric than that of decibels.
Music set to blastbeats then tends to not actively sound fast like a runner, it just sounds rapid, like a stream. Death metal, a production of thrash and therefore very concerned with conveying speed, used to deal with this problem by using the blastbeat sparingly. In the context of surrounding polka beats, straight rock and roll beats and double-bass rock beats, the sense of propulsion could be retained. Morbid Angel on their seminal debut, employ this technique. Blastbeats are sparingly used and even then their beginnings and endings are very prominently accented so the listener is always clued in as to the where and why of the composition.
However a slew of latter day death metal bands pursued extremism for its sake and their music is often described as a blastfest. The end result sounds barely controlled, or often outright chaotic. This is used to positive effect by some crafty musicians, however.
Black metal music, which came into prominence in direct contradiction to the popular death metal paradigm of the time, consciously employs long streams of blastbeats in a different way. They slow them down so that their bareness and simplicity is audible and they do not often accent the beginnings of bars. The end result is active but curiously still. Propulsion is sacrificed for constant and equally applied force. Furthermore the usual sound design of black metal is that of obfuscation in washes of distant-sounding reverb. It is in this way that black metal music strives to achieve a sense of trance, much like certain types of electronic music. The kick drum especially is of much less priority in black metal, to the effect that sometimes it seems one is listening to a sea of snares.
Black metal's trajectory towards trance and hypnosis lead to the logical conclusion of rhythms built on drum machines, which can play completely evenly and for as long a time as required and never quit your band to go join someone else's more successful outfit. This ties in with how much black metal is made by solitary individuals handling all instrument, vocal and programming duties, and goes some way to explain why there is so much black metal out there.
More terms to be dissected as they come up.