(Note: as of 11 July 2006, this policy is incomplete and is not yet enforced. Feel free to comment below with any complaints or requests for clarification.)
There are many roleplaying communities online that specialise in anime or celebrity characters; this is not one of them. This also isn't a fandom roleplay -- you can play a Jedi Knight if you want, but not one who is copyrighted (unless you're the copyright holder). Original characters only (but note the White Wolf exception below). If you must play Lestat or a dragonrider of Pern, do it somewhere else.
What's that leave? Pretty much everything else you like. Vampire, pixie, human, dragon, cyborg -- it all goes.
All characters are expected to have writeups in the Twine Encyclopaedia, which must abide by the Twine copyright policy. These can be as lengthy or as brief as necessary to explain your character. You don't have to apply to play a character and you don't have to get your character approved, but you should supply a Twine entry for your character with some basic info. If you're writing up a new character, you probably won't have a thick backstory yet, so your entry will be shorter than some. That's not a problem. (The "lengthy" entry referenced above is for a character played more or less continuously since 1999 -- hence the length.)
Twine has a guide to help you get you started. You probably want to note some basics in your writeup: character name (which will also be the entry name), species, gender, height, build, and colouring help detail characters for your fellow writers. If you're not sure of something yet, it's okay to leave it blank until you know -- for now, the point is not to list every detail, but to give the others something to work with. The more you deviate from human norms, the more you have to explain -- saying your human male is of average height will bring up a pretty good estimate in the minds of most people, but saying your female elf is tall could mean four feet or seven, depending on who you ask; saying your dragon is "average size" means nothing more than "really really big, probably."
Members may play White Wolf characters in freeform roleplay if they also play original characters. However, don't play only WW characters here. There are other boards for that. Make some originals -- try it, you might like it. Ideally, members should have at least one original character active for each WW character.
WW characters should have entries in Twine and are encouraged to note in their entries which system the character is from (e.g, "Millie Carr is a Vampire: The Requiem character") and that the system is copyright White Wolf, Inc. Clan and race information from sourcebooks should not be added to Twine, but a link to a topical Wikipedia entry or other site is a good idea. Such links help members not familiar with the system to be aware of your character's quirks, whether that's Malkavian madness or Black Fury sexism.
In the event of a White Wolf game being run on-site, character sheets will posted to the message board, not Twine, to comply with the Twine copyright policy.
Best of luck with your characters! They're bound to be a handful (mine are!).