Most people know that they all sound different (duh, lol). But very few people know where all the music evolved from, or how. Here I'll go over how bands like Black Sabbath got their sound, all the way to Children Of Bodom, Megadeth, and yes sadly Metallica.
Having no better name for it, bands like Black Sabbath, AC/DC (ok now true metal there but you get the idea), and the like fit in here. Most of these bands got their sound from early Rock 'n Roll, blues, and jazz (very slightly on the last one). Black Sabbath actually is hailed as the creator of Metal, and before "creating it" they played 12-bar blues in bars in england (seriously).
Brittish Heavy Metal
Later on them crazy Brits inspired by like likes of Black Sabbath began to emerge. Judas Priest, Iron Maiden etc... Mainly they were more musical and melodic, and invented the idea of two guitar players playing seperate parts and harmonizing.
Bands of the second wave are much lesser known... Diamond Head one of Metallica's influences being one of the better known. Not really much difference except they took twin guitar parts up another step having a dedicated lead and rhythm players (though they sometime switch duties in the song, which is still done today).
FAST... loud... aggressive took the lessons learend by Brittish Heavy Metal and second wave bands, combined it with the speed of hardcore punk (yes metal has its origins in punk... live with it) won't give any examples because most of these bands evolved to thrash later on anyway.
Speed metal soon turned into thrash, thash took the same ideas but added technicality to the songs (in some cases), and giving more leeway for tempo changes and long solos. Examples... pretty self explanatory. (80s) Metallica was probably the best known thash band, along with (80s) Megadeth, and Slayer. Anthrax was deeper rooted in Speed and Hardcore since they came from the East Coast.
Second and third wave thrash bands like Teatsment, Exodus, Overkill, Death Angel, Canada's Annihilator, and believe me... countless others drew upon their influences and added in some cases.
Mainly influenced by dark thrash like Slayer came about in the early 90s... and yes Venom was earlier but as a general rule. Death is considered by many to be the true fathers of the genre (hence the name). Cannibal Corpse *cough*crap*cough* (no offense to their fans), Morbid Angel, and a slew other other bands (only genre I can think of with more than thrash). However the genre got so popular a lot of bands that really weren't that great were getting signed in the 90s, making it a lot harder to find good stuff... I'm sure all these bands have their fans though (I try not to judge).
Second wave stuff either goes in one of two directions, adding a hint of progressiveness to their music, or amping up the aggression to the next level. A few first wave bands redefined their sound like Death to be more melodic while others went the opposite way. I like both but go check them out for yourself.
Also branched off of thrash basicly the European form of Death Metal. Sometimes more melodic, other times more violent. Bathory, Emporer etc... like Death Metal their music thrives on the chaos you can create with instruments (not a bad thing).
Second wave bands here sadly took much of the music waaaaaaaay too seriously. From church burnings to murder... they got it all.
Death and Black Crossover
Children Of Bodom being a well known one. Not having the seriousness of their forebears but the melody and aggression remaining they can often be one of the more insteresting genres.
Taking all the lessons learned so far and infusing it with Prog Rock, these bands came about. Either being extremely melodic, technical, or catchy. Dream Theater, later Death albums, Control Denied (rest in peace Chuck), and many others.
Not that popular... but very cool stuff. Takes Progressive another step further. One guy playing guitar for a whole cd... can it get any better! VERY VERY Technical stuff. Yngwie, Vai, Satch, or my personal fav. Michael Angelo Batio (awesome guy saw him live.... godly). Borrows from jazz, blues, anything!
This is just an overview... now go and listen to METAL!