What is a user account?
When you signed up here, you created a user account. Yours is "Ninnie" :)
Great question! What is the purpose of the fraudulent ones?
Mostly to post forum spam promoting the same kinds of products you see promoted in e-mail spam.
how do you tell a legitimate user from a fraudulent one?
It's a never-ending battle, and there aren't any specifics. But most of the time it becomes clear right away. When someone registers and starts posting messages about, say, a "great new credit card offer" or something like that without ever posting anything about quilting, you usually know right away.
But so far we've been fortunate enough not to have many of those.