Usually warped brake rotors. You probably need them replaced.

Probably a locked caliper. I would get it checked out and then go home and replace it if you know how

either the pads or shoes are slipping or the rotors or drums are warped or nonparallel.

http://www.stoptech.com/technical-suppor...

http://www.cleanmpg.com/forums/archive/i...

Yes, worn discs and/or pads. When you change the discs, change the pads too.