it sits on the input shaft of the g/box and when you put the clutch pedal in it presses against the pressure plate. depending on whats worn you may not need a new clutch you may be able to replace just the bearing but if you're replacing the bearing check the condition of the clutch plates and pressure plate of course!

also.... if it is the bearing only, as the box has to come out to replace it, do you want to go through it all again at a later date to replace the clutch?? ( ie: another few hours labour to pay for )!!