Ditto is a Normal-type Pokémon.
Ditto is a light purple / magenta free-form blob with vestigial facial features. It also appears to have two vestigial, handless 'arms' and two 'legs' protruding from its body.
When two wild Ditto meet, they will attempt to transform into each other.
Ditto will also transform into a rock when sleeping to avoid being attacked.
Ditto is capable of transforming into an exact replica of any physical object or Pokémon and copy their moves.
If Ditto tries to transform into something based on memory, it may get some of the details wrong.
Occasionally, Ditto will be unable to change its face initially. They can get over this issue.
Some Ditto are unable to change their size.
Ditto will also be unable to remain in a transformed state if it starts laughing.
Ditto transforming via Transform cannot copy the stats of another Pokémon. Ditto transforming via Imposter do copy all stats except HP.