the clutch slipping usually only occurs when your at high rpms and the clutch cant hold the flywheel. the friction points on the clucth disc get worn out and there is really no friction. the engine will continue to rev, and the car might even slow down due to the drop of power. then it might catch all of a sudden and the rpms will drop bback to normal.. that is when the Clutch on a Manual trani is slipping. much the same FEELING occcurs when an auto trani slips, you'll see the rpms rise and realise that the car isnt going any faster, and then they might drop and you might feel a little jolt as they resume normal function.
neither is good, and both are a bitch to fix. good luck