Egg Groups, With these you can help to make your Pokémon breed...your Pokémon can only breed with Pokémon in the same Egg Group as them, however severall Pokémon are in 2 Egg Groups. Anyway, Here's a List of the Egg Groups with pictures of all the Pokémon in it.

Remember, our friend Ditto can breed with any Pokémon except those in Group X:

Group X: Cannot Breed
Group 1: Plant

Group 2: Bug
Group 3: Flying

Group 4: Humanshape
Group 5: Mineral

Group 6: Indeterminate
Group 7: Ground

Group 8: Water 1
Group 9: Water 2

Group 10: Water 3
Group 11: Monster

Group 12: Fairy
Group 13: Dragon