Variable location and related information

[General Ray]

I need to know a little about the $vbulletin variable. First off, what is this variable? Where is it called? How does it work? I need to call a specific custom db query for a user, and I see that this variable is used for these tasks (Such as username, userid, password etc). It's really starting to piss me off that I can't find the page this variable is called in.