The brushes are in the motor - this cheesy video shows how to change them...
http://www.youtube.com/watch?v=0GmtRBiXLFc
Brushes are made of carbon and deliver electrical power to the rotating part of the motor. It is normal for them to wear out over time and to need replacing.
When brushes fail the motor starts to run roughly and it gets hot and smelly - you notice it first when the motor is working hard like on maximum spin speed.
Given that your motor runs fine when not spinning the drum but does not even try to turn the drum around when it should be spinning it my guess is that you do not have a brush problem.
I also guess that the people who say the brushes need changing are just guessing like me - but I'm often wrong and brushes are quite cheap.
Peccavi
January 2009