HM

HMs stand for Hidden Machines and unlike TMs they can be used more than once.

Generation 1
In Pokemon Red, Pokemon Blue and Pokemon Yellow there were only 5 HMs: Usuable with Cascade Badge In battle slices the target Field Action cuts down small trees.
 * HM01-Cut: Obtained from Captain of S.S Anne after beating your rival.

Usable with the Thunder Badge In battle flys up first turn avoid all attacks apart from gust and thunder which do double damage Field Action flys to town/ Pokecentre already visited
 * HM02-Fly: Obtained west of Celadon City past Snorlax above cycling road.

Usable with the Fushcia Badge In battle surfs on a wave which crashes into the enemy Field Action able to move on water
 * HM03-Surf :Obtained somewhere in the safari zone.

Usable with the Volcano Badge In battle builds up strength to unleash on target Field Action move big boulders
 * HM04-Strength: Obtained by returning Old Warden's Gold teeth from the safari zone.

Usable after the Boulder Badge. In battle blinds the enemy to reduce accuracy Field Action lights up dark caves.
 * HM05-Flash: Obtained from Professor Oak's Aide on route 2 (go through Diglett's Cave) once you have got 10 pokedex entries or more.