Physical Description:
This not only gives you a visual image to match up with the character but it could also play an important role in combat scenes, etc. For example, if your character is over 6ft tall this would play out differently than a character battling a dwarf. Also, if you use a graphic or avatar for your character, these descriptions should match up as closely as possible. If you give your character black hair, then use an avatar that does in fact have black hair.
Background:
This is the point where your story telling abilities come in handy. Your background is what gives you, and everyone else, the most insight into your character. Where were you born? What was your childhood like? What are the significant events in your life? What skills have you learned? What bad habits have you picked up over the years? Basically think of this as if you were writing a biography of a real person for a magazine, what sort of things do you think would be important?
Current Status & Motivation:
This is a key to your character's behavior as well, you need to know what the key factor is that has brought you to this world or this point in time and what is currently motivating your RPG character. Are you looking for fame and fortune, setting out to save your planet, or searching for purpose in life? Your current status and motivation will play a big role in how you behave during game play and what roles you will pursue.
Game Relevance:
Lastly, be sure to add any information relevant to the specific RPG. Are you a vampire? Then when did you turn and who was your sire? Are you a knight? What motivated you to become a knight and who trained you as a page?
While you may have trim the fat from your actual profile when you post it on a game site due to size limitations, the more detail that you have about your RPG character the easier it will be to play them in a realistic fashion.
You may also want to be sure that you are up to speed on some of the traditional gaming and battle vocabulary, so check out these handy articles:
General Gaming Terminology
Battle Terminology