A description is supposed to tell other players what they see when they look at you - what clothes you are wearing, what items you are carrying, perhaps any markings you might have. You can also describe the way your character carries him or herself (hinting at personality), but it is supposed to be all visual. No character history that others can\'t know just by looking at you, no little tidbits or random facts here and there - because other characters can\'t know that without talking to your character first.
Sometimes descriptions are really heplful for mentioning little OOC things about your character to help others, such as [ OOC: I am playing a slightly telepathic character, so your help when roleplaying would be appreciated ] or [ OOC: This character is very aggressive and likes to challenge people to duels. I will always ask before, but don\'t think I\'m a random spammer please ] and etc.
Can\'t think of anything else at the moment :)