This guide states most of the ROM letters used in GoodTools renaming.
If your game does not state any of these (most ROM sites remove the letters or such, or in the case of Emuparadise, extend the region, revision/version and languages and delete the rest), then get GoodTools and you can see what ROM you have downloaded.

All experienced hackers should know these already, this is for new hackers who have started, or people just curious of all the extra tags in the ROM filename.

* = in place of a number. This is the number of the type of ROM so the [b1] would be the first, [b2] second bad dump made... you get the idea?

ROM Dump Status Codes

[!] - Verified Good Dump. These are the exact same as the original version as reported. Obviously, this is the best option.
[a*] - Alternative. Rereleases to fix bugs, added lockout chips, or to terminate the use of Game Genie, for examples.
[b*] - Bad dump. These have errors in the dump rendering the game unplayable either altogether or to an extent. Avoid these altogether.
[f*] - Fixed dump. If the normal one runs like crap on an emulator or multicart/flashcart/etc, then unofficial fixed ROMs can correct this.
[h*] - Hacked dump. States that the header has been modified perhaps to bypass lockout chips or if the header was changed for a ROM hack as we all know what it is.
[o*] - Overdump. Random rubbish data at the end of the ROM, that doesn't do anything at all to the game.
[p*] - Pirate. Means the same thing as (Unl) but [p] is more common when the dump is from a pirate copy of a retail game - usually the title and copyrights have been removed or replaced with differences.
[t*] - Trained ROM. Cheats were added to the game, which can be selected on a menu before booting the game up properly. This might have been done in the past to bypass Game Genie blocks.
[!p] - Pending Dump. There's no dump that's verified but this is the closest possible one. GoodTools didn't have this until a while ago so most pending dumps are just left unmarked.

Country Codes

(A) - Australia
(As) - Asia
(B) - Brazil
(C) - Canada
(Ch) - China
(D) - Netherlands
(E) - Europe
(F) - France
(G) - Germany
(Gr) - Greece
(HK) - Hong Kong
(I) - Italy
(J) - Japan. Second recommendation for ROM hacks.
(K) - Korea
(Nl) - Netherlands
(No) - Norway
(R) - Russia
(S) - Spain
(Sw) - Sweden
(U) - United States/North America. First recommendation for ROM hacks.
(UK) - United Kingdom
(W) - World - however this actually means J/U when E has no differences to the other two versions whenever it's used.
(Unl) - Unlicenced. [p] means the exact same thing, though.
(PD) - Public Domain
(Unk) - Unknown Country

Additional ROM Codes

(REV*) - Revision Number. Usually used on NES and N64 games.
(V*.**) - Version Number. Usually used on SNES, GBx and GBA games.
(M*) - Amount of languages found on the menu when you start it up. I noticed though, this is usually on Game Boy and GBA games, for some reason.
[T+*] - Most Recent Translation
[T-*] - Old Translation, now obsolete
(Alpha/Beta/Prototype/Pre-Release/Kiosk Demo) - Pre-release games, usually!
(Hack) - ROM Hack. Might be preceded with the game name's abbreviation before the word Hack.
(Menu) - Pirate Multicart which uses a menu and not a reset dipswitch.
ZZZ_UNK - Unknown Classification. A ton of the Goodsets' ROM hacks attach this prefix.
(*MBit) - Megabits of the ROM size.
(*k) - Kilobits of the ROM size.
(PAL) - PAL Display.
(NTSC) - NTSC Display.

The system specific letters aren't here, but I'm too lazy to put those on there right now anyway.
Thanks to the Wikipedia GoodTools page, which lists all of these in the same order (couldn't get a copy of a GoodTools codes txt anywhere).

____________________