Illusion will change the appearance of the Pokémon to a different species. This is dependent on the last Pokémon in the player's party. Illusion will replicate the type of Poké Ball, species name, Shininess (if any), and gender of the Pokémon it is masquerading as; however, it does not replicate the level of the masqueraded Pokémon. The effect is only aesthetic, and does not affect the Pokémon's types or moves.
Illusion is broken when the user is damaged by an attack, but it is not broken by Substitute, Life Orb, weather conditions, status ailments or entry hazards. It is also broken if the user's Ability is changed or nullified.