DISCLAIMER: I personally don't believe in categorizing music in genres and zillions of subgenres, if it sounds good, listen to it. But if you want the definitions for the bands you mentioned, I will help you with that.
QUOTE(sshd @ Oct 30 2003, 06:05 PM)
Some 10 years ago Black Sabbath, Ozzy Osbourne, Iron Maiden and GNR were considered Heavy Metal.
Black Sabbath and Iron Maiden belong to the elusive "New Wave Of British Metal" genre, whatever that means. I consider them as regular Heavy Metal, or possibly Hard Rock. GNR is glam rock.
Black Sabbath's and Iron Maiden's influence towards any modern metal music is so very strong that they should and they do get all of the respect from all metal music nowadays.
Metallica is sometimes considered as Trash Metal instead of Heavy Metal because of the differences in style. Today's MTV "nu-metal" basically rips Metallica with the morse code style riffs and palm-muted power chord structures.
Can you see the irony that nowadays Metallica rips nu-metal bands

QUOTE
I prefer what some call Goth Rock or Gothic Metal: Theatre of Tragegy (old), Within Temptation, Nightwish, Gathering (old), Moonspell, Sins of Thy Beloved, Therion, Tristania, etc.
Those are all very good bands. As an interesting side note, Therion started as brutal Death Metal, but later on included the choir and started playing some very melodic Hard Rock (Iron Maiden) -style riffs. In terms of musical prowess, modern Therion can be considered as classical music. If some techno or pop rock boy starts babbling about metal's horrible guttural vocals and simple two-note riffs, play them some Therion.
BTW, have you already heard the Summer Night City cover by Therion? Proves that Abba
did make some pretty technical music

QUOTE
I also like metal like Alastis, Samael, Paradise Lost, Tiamat and Rammstein, but I have no idea what sub genre they belong to.
Death metal. Alastis belongs to the "melodic death metal", or Gothenburg-style genre started by In Flames. Paradise Lost started as Death Metal, but changed their style to regular pop rock later on.
Other
extremely good Gothenburg-style bands include:
In Flames (obviously)
Amorphis (early)
Arch Enemy (all)
Dark Tranquillity (later)
The early productions of Samael, one of my favourites, can be considered as Black Metal, although musically they were Death Metal. Nowadays they have their own genre, probably Industrial Metal, as well as Rammstein.
Edit:
Typos