Serial key

Unlock key (a.k.a. registration key) is used by shareware or other "limited" games to unlock the full game. There's very little difference to the other key types than the fact that you actually have the full game already, but can't play it in full without unlocking/registering it first.

This can function as a form of DRM by preventing play as long as no correct key is provided, in some games this limitation only affects singleplayer or multiplayer but not both (commonly the latter is blocked if not both).