First off metal is an umbrella term (which means it can mean lots of different things) in general people think of it as hard rock. The problem with sheltering all bands of the "metal" genre under that same name is that when you ask a huge Slipknot fan they'll say its one thing but if you ask a Judas Priest fan they'll say something completely different.
Metal in the early to mid 1970's is generally called classic metal or hard rock. Bands that fit this category are Led Zeppelin, Aerosmith, Black Sabbath, and Jimi Hendrix. Other than Black Sabbath all these bands had a more bluesy sound than what is considered "metal" but they are in fact hard rock/heavy metal.
Than in the late 70's a wave of British metal that had the dark lyrics of Black Sabbath but with less repeditive and slightly more meladic riffs. Bands like Iron Maiden, Saxson, Demon, and Judas Priest. All these bands had the technical ability of the earlier metal bands but used the energy, aggression and speed of many punk bands at the time. This type of Metal is generally called The New Wave of British Heavy Metal or NWOBHM.
At the same time another Metal movement was beginning. Bands like Def Leppard and Van Halen were using incredible guitar-virtuoso leads along with slower meladies made it more mainstream than the NWOBHM bands. There vocals often including high falcetto screams.
In the early 80's the L.A. scene picked up and overnight many bands with the same formula as Def Leppard and VH, became the next wave of mainstream rock and metal. Bands like Motley Crue, Ratt, Ozzy Osbourne and Quiet Riot added incredible stage shows along with a cross-dressing look of Glam Rock took Metal to new popularity heights. That type of metal is usually called Hair Metal or Pop Metal.
In 1986 a San Francisco band named Metallica released Master Of Puppets and another type of metal was born. They had the guitar of the NWOBHM but souped up the tempo and used an even more agressive playing style. They had screaming, enraged vocals and solo styles much like the L.A. scene although they had a much more serious style of music and dress. Soon after Megadeath, Antrax and Slayer followed suit. This metal is generally called Thrash Metal.
By the late 80's both scenes had become warn out. But in 1988 a single was released that would change metal for the next few years, that single was Sweet Child O' Mine by Guns N' Roses. It had they hair metal style of play and they assosciated themselves with those bands but had a bluesier and more serious style. Sweet Child O' Mine proved that Metal bands could show there softer side. Sweet Child O' Mine also came off of the best selling debut album ever: Appetite For Destruction (more than 15 million sold)
When Grunge came to the rock spotlight in the early 90's both scenes needed another miracle. Neither got it. But in 1991 a new band hit the spotlight. Pantera had Metallica-like music but they were even more brutal and heavy than Metallica. They played faster than any metal band before them, even Motorhead. They had the aggression of punk and but Dimebag Darrel had the virtiuso ability that often crowned him "The Next Eddie Van Halen". In 1991 there single debuted #1 beating every grunge and metal single at the time, even Enter Sandman.
In the mid-90's a new metal had taken over. This time took every page from Panteras play book but then added in the influence of Rap. These band generally called Nu-Metal used scratch effects and Rap-style singing with the crushing music of the Pantera's and Metallica's before them. Although this type of music is frowned apon by fans of just about every other type of metal, it is still a type of metal and deserves to be on this list as much as anyone. Bands from this movement are Slipknot, System Of A Down, and Papa Roach.
In the early to mid 2000's nu-metal still reigns king of Metal, but with harder underground bands like Mastadon hitting the spotlight (and playing Ozzfest this year on the second stage) it obivously wont be long until another group of bands claim there own place in Metal history.