The clutch release bearing, also known as the throw out bearing is located in the clutch assembly behind the pressure plate and is around the input shaft of the transmission. The clutch release bearing is either mechanically or hydraulically controlled. The hydraulically controlled throw out bearing will have ether an external clutch slave cylinder that pushes against a release arm, or an internal hydraulic integrated bearing assembly. If the integrated hydraulic type of bearing is leaking, then you will need to pull the tans mission to replace the unit.
An indicator of a bad release bearing is that when you put light pressure on the clutch pedal you will hear a grinding or a rubbing sound. This is due to the bearing part of the unit failing. You will need to replace the bearing immediately as there is a high likely hood of the throw out bearing coming apart in the bell housing causing damage and disabling the clutch.