Userinfo
Staging ground for colorbars, self-praise, and general wank. It displays your default icon, as well as any personal information you have chosen to let people laugh at. Some people consider reading userinfo a form of Internet stalking.
Userinfo varies greatly between LJ users, from the absurdly concise to endless reams of text and images. It is primarily accessed only by lamers, and/or during a flame war.
I can't believe they said that...
Often, on their userinfo, people who think they are an internet celebrity will have testimonies about themselves from other people they think are internet celebrities. Sometimes these testimonies will be negative, in order to prove that the LJ user can laugh at themselves.
This is a lie. The seriousness with which an LJ user takes the Internet is directly proportional to their need to prove how unseriously they take the internet. These users will still sob into their pillows once you tell them that they don't have any friends.
Communities
Userinfo for a community is effectively the same as for a user, but will generally include a statement of intent, the moderator(s) journal(s), and the rules which you can break in order to get banned. Community userinfo is never funny or interesting, regardless of how serious the community is.