Variable location and related information
 

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.

$vbulletin is the registry object of vBulletin. It contains a lot of what you'll ever need (e.g. input cleaner or database object). It is created in init.php, which calls upon Class_Core (Core also calls other classes).

This is exactly what I need to know. Thank you Dismounted.
Is there anything else I should know on this subject?

There's a lot of things, I guess, like how to use the input cleaner, and DB class, etc., but these are already covered in articles that have been posted.