HM

HM's or Hidden Machines are in almost every way identical to TM's, but for three differences: the moves cannot be overidden by simply learning another move, the moves can be used outside of battle after obtaining the relevent gym badge and the HM can be used multiple times without breaking. Additionally, pokemon on a third generation game cartridge cannot be transfered to a fourth generation game card if it knows a HM move, you must go to the Move Deleter in your game in order to transfer these pokemon to Pokemon Diamond/Pokemon Pearl.