From Bulbapedia, the community-driven Pokémon encyclopedia.
Egg cycles are a number that determines how long it will take for a Pokémon egg to hatch. The value of the amount of egg cycles remaining until the egg hatches is stored in the same number that the happiness value is for normal Pokémon, however, rather than counting up after 255 steps have been taken, it counts down toward hatching on reaching 0.
Individual Pokémon species have a value for the amount of cycles an egg of theirs will take to hatch when received by default, however, the cycle's value, if altered, can be as high as 255 for an individual egg.
Please note that not all of these species can be legitimately hatched from an egg, and that all steps are the minimum numbers as of Generation IV.
List of Pokémon by base egg cycles
Generation I families
Generation II families
Generation III families
Generation IV families
Generation V families