Most think only of manual transmissions when talking about the clutch, but automatic transmissions contain clutches also. Clutches allow for us to smoothly put the vehicle in gear by controlling the slippage between the gears. This is necessary because the engine is in a continuous spinning motion while the transmission is not.
How do you know if you need a new clutch? A worn clutch will slip as you apply power. Meaning, the engine speed will increase greatly when accelerate, even though the clutch pedal is not pressed. A good clutch will lock the engine to the transmission, so its speed is directly linked to the changes in the vehicle speed.