Informal short form for "boyfriend".
Often used on IRC and other realtime online fourms where people try to save keystrokes.
Refers to someone's male significant other more often than someone's friend who is a boy.
Not that this says anything about our society . . .

Also an abbreviation for the Burned Furs, a group of people who took up a (mostly) good cause in furry fandom, but due to major faults in the presentation of their ideas ended up raising everyone's hackles.

For some odd reason their founding documents take a vehement stand against veganism, informed by some rather odd dentistry.