The Rake ability allows you to make special attacks called rake attacks that are performed
- as part of a Pounce attack, or
- sometimes (see below) as part of an Improved Grab attack, or
- in addition to a natural attack as an Attack Your Opponent grapple action, only if you were grappling at the start of your turn.
(Intermezzo: Not all monsters with Improved Grab allow you to use Rake. In the Monster Manual the monsters that do are: Hellcat, Leonal, Leopard, (Dire) Lion and (Dire) Tiger.
Examples of monsters that have Improved Grab and Rake but don't allow this are Annis and Behir.)
Anyway, the rules for Grapple say:
Some of these actions take the place of an attack (rather than being a
standard action or a move action).
This is the case with Rake for Attack Your Opponent and arguably also for Pounce and Improved Grab, so I'd say a rake attack is a special attack that is part of an action and not an action itself. Compare this also to the extra attacks granted by the Two Weapon Fighting feat or the Haste spell.