User becomes a copy of the target until it leaves battle.

Transform causes the user to become a copy of the target (copying image and moves).
Transform bypasses accuracy checks to always hit, unless the target is in the semi-invulnerable turn of a move such as Dig or Fly.
It is not affected by type immunity.
Transform fails against:

• Shadow-type Pokémon
• Pokémon that are protected by Crafty Shield
• if the user is disguised by Illusion (not in Gen 1)
• if the the target is disguised by Illusion (not in Gen 1-4)
• targets that are behind a Substitute (not in Gen 1-4)
• Pokémon with the ability Good as Gold

A transformation can not be passed by Baton Pass and the user will return to its original state after switching out, fainting or when the battle ends.
Transform also copies some of the traits of the Pokémon including:

• Species
• Appearanace
• Types
• All base stats, except HP
• Current stat changes
• Current moves
• Height and weight
• Catch Rate (only in Gen 3-4)
• EV-yield and base experience (not in Gen 1-2)
• Ability
• Cry sound (not in Gen 1-3)
• Gender (not in Gen 2-4)