Ditto, a Transform Pokémon. It is able to rearrange the cells of its body and assume any form. Its only attack is Transform.
– Ash’s Pokedex
Ditto is a very unique Pokemon. It can only learn one move, Transform, which makes it transform into the Pokemon it’s fighting with, and copies its moveset. I think if a Ditto tries to Transform into a Ditto, it might fail.
Ditto is not very useful for battling, but is most useful in breeding (making them the sex slaves of Pokemon). Some Pokemon (like ones with only male or no genders) can only be bred with Dittos. It was very inconvenient in Gen V without having access to Dittos at first. If you have Ditto hold an Everstone, there’s a greater chance for the Ditto’s nature to be passed down (50%), so it’s useful to have Dittos in the natures you want.
Ditto looks like a purple blob. Its face is very amusing. In the anime, there was a Ditto that can copy Pokemon’s forms, but not their faces, so it ends up with Pokemon with Ditto faces, quite funny-looking.