Bit characters are characters that have bit parts. Meaning, they aren't the focus of the story, but appear, perform actions, and may have lines.

A good sign of a bit character is that they are in the story, and aren't background characters (even if they aren't named), but the story isn't 'about them.'

They may or may not have description-- sometimes they don't need any, and they're left to imagination. It would be ridiculous to describe every character your leads see... especially when the leads don't really take notice of them themselves. How many people a day do you see that you can describe only as 'a security guard' or 'a mailman' but not give a detailed description?

Of course, if things are a little TOO-lacking in description, it all becomes beige prose. :
Even so, that's a bit character. I know that's probably covered on the wiki, but I needed to get my daily dose of blathering over with. >_>;