From Bulbapedia, the community-driven Pokémon encyclopedia.
Moves are passed down from a Pokémon breeding pair to their offspring, in the following ways:
- If both parents know one or more moves that their offspring could learn by leveling up, the offspring will have that move.
- If the male Pokémon knows a TM or HM move that the offspring can learn, the offspring will have that move.
- If the male Pokémon knows a move that the offspring can learn only by breeding. For example, if a male Dragonite knowing Outrage is bred with a female Charizard, the resulting Charmander will know Outrage.
- All offspring Pokémon, in the absence of passed-down moves, will know at least one move when hatched, labeled in their level up move list as either "Start" or "--" in lieu of a level number. If these moves, together with the other egg moves obtained in the above ways, number more than four, then the moves on top of their level up move list (with "Start" or "--") will be the ones that do not show up.
Egg Moves are most commonly viewed as moves that fall under the third category. Most online Pokdéxes list moves from the third category as a Pokémon's Egg Moves. Before breeding a Pokémon, one must consider whether or not to use any Egg Moves in that Pokémon's moveset, and breed accordingly. To learn more than one Egg Move, since they are only passed on by the father, only a father Pokémon in the appropriate Egg Group that can legitimately have all of the moves in one moveset can be used. This makes certain combinations of Egg Moves impossible for certain Pokémon. Eg. Tentacruel has both Rapid Spin and Mirror Coat as available Egg Moves, but no Pokemon in the Water 3 Egg Group can learn both moves so Tentacruel can't use both in the same moveset.