In its natural state, Ditto is a purplish(bluish in Amelia's case) free-form shape blob with vestigial facial features. It also appears to have two vestigial, hand-less 'arms' protruding from its body. However, it can take the form of anything, even inanimate objects.
Ditto is capable of transforming into an exact replica of any other Pokémon (an ability that is also shared with Mew), or indeed any physical object. However, if Ditto tries to transform into something based on memory, it may get some of the details wrong. Ditto will also be unable to remain in a transformed state if it starts laughing. Frequently, a Ditto appears that can't transform its face. In rare cases it will lack the ability to increase or decrease in size, thus rather than creating exact duplicates, it will create (usually) miniature versions. Ditto cannot mimic the strength of another Pokémon. Ditto's ability to breed with any non-legendary Pokémon is also useful for creating Pokémon eggs of rarer Pokémon that it may be difficult to get a mate for, such as the starter Pokémon.
It's ability, Eccentric, allows it to Transform into its opponent as soon as it enters battle, copying the opponent's stats exactly, with the exception of HP. Eccentric copies all stat changes on the target originating from moves and abilities such as Swords Dance and Intimidate, but not from items such as Choice Specs. Eccentric will not Transform the user if the opponent is an Illusion or behind a Substitute.
Eccentric does not copy its opponent's IVs but it does copy all EVs except for HP.