.:Metalgenres:.
Instead of putting a whole pile of text about the different kinds of metal genres (as I did before), I'll give you some links to websites providing the info you are looking for. Metal keeps changing and more/other info pops up now and then, so it's no use in my opinion to keep updating this page with new text when there already are some great sites offering the same information. ;-)
Wikipedia – Heavy Metal: history, evolution, subgenres, etc..
Wikipedia – Heavy Metal timeline: a little timeline about Heavy Metal (albums, bands, ...)
Wikipedia – Genres+Musicians: an extensive list of genres with the appropriate bands
Wikipedia – Heavy Metal subgenres: a list of the different kinds of metal genres
Wikipedia – Heavy Metal fashion: some info about Heavy Metal clothing
Antenna.nu – Heavy Metal genres/styles: a description of different genres and styles
Anus.com – Heavy Metal styles/techniques: an overview of different styles, used techniques and 'sounds'
Anus.com – Heavy Metal genres: an overview of subgenres
Anus.com – The history of Heavy Metal: a historical overview of Heavy Metal
Doom-metal.com – Doom Metal: site dedicated to Doom Metal, offering info about the genre (see 'Information') and more.