One of my roleplaying sites -- Aparecium, a Harry Potter roleplay (hush. I like it) -- along with some of the other roleplay sites I've visited, have a template to complete before one can become a full time member. A character template, a guideline for one's character development. Here's Aparecium's character-sheet (lightly edited).
BasicsAs a note, Aparecium's word count "guides" are just that. Preferably, we suggest you have more (as more is always impressive), and, generally, most people go either over it by a bit, or go over it by so much that one doesn't even want to read it. As in... 1461 words for a personality of someone's character, or a friend of mine deciding to put six-thousand words for his entire background (we died a bit inside to see it). How long it should be is [generally] up to you, however much you know about the character, and however much you care to share.
Date of Birth;
[code][ Celeb Name|Character Name ][/code]
Insert example here. 400+ words.
Other applications get a lot more detailed, but don't have word counts, such as the following:
full name: answerOkay, so maybe that was a little too detailed, a lot of "I-don't-really-want-to-know" things. What is a good medium? It's up to you, as the creator of the character, to decide. The best way to do it, I find, is to figure out what you would like to know about a person if you were meeting them in person. What traits of theirs is important to note? What characteristics would attract/repulse you in a person?
hair/eye color: answer
clothing style: answer
likes: at least ten
dislikes: at least ten
random facts: answer
personality: two paragraph minimum
sexual orientation: answer
turn ons: answer
turn offs: answer
significant other: answer
history: three paragraph minimum (read the rules to check acceptable paragraph length)
How very self centered. I know. But hey. Whatever you want to know about someone, I bet someone else is just as curious about it.
Most of these are fairly self explainatory. Actually, most of the second template explains the first: Appearance, Personality, Personal/Family Background. Appearance, obviously, they want to know what your character looks like. How tall/short is she? What color hair/eyes? Scars, piercings, anything that distinguishes her appearance? A blue eye and a brown eye? Personality. How do they act around friends, around strangers, or even alone. What do they like/dislike/fear? What goals do they have in life, what secrets may they be hiding (yes, your character probably has a secret, even if you don't know it -- most people do!). Even which way they swing, if you care to add that. Background -- where do their family come from? How was your character's childhood? What was it like? Background is often very important to explain how/why a character acts the way they do. [Such as my Lynn. She's utterly terrified of being alone with a man in a room because her stepfather abusing her as a young girl.]
For me, a character sheet is a way to help me learn more about my character and what makes her tick. In doing so, others can learn more about a character, and maybe want to help you develop it through roleplaying.
With all of this in mind, I really ought to go and finish Nattie's personal history and iron out the little kinks... Maybe she'll end up killing her mother's murderer, which is what brings her to be so dispassionate about killing -- the "I've killed once, I might as well kill again" mindset.
Good luck with your own characters!