Pokemon HeartGold/Pokemon SoulSilver - TMs/HMs

TMs (Technical Machine) and HMs (Hidden Machines) are items the hold moves that you can teach to Pokemon. TMs can be found while traveling, bought from a store and some are given to you by gym leaders after you defeat them. You are only able to use TMs one time before they disappear, unlike HMs which can be used unlimited amount of times. Certain TMs can be used outside of battle like Flash which lights up caves and all HMs are usable outside of battle. Certain TM moves can be given to offspring if you set the right conditions when breeding.