If a player "claims" a key, the game is added to their library, as if they had purchased it. Revoking the key does not remove the game from their library. (I'm not sure, but I think that once a key has been claimed, it can't be revoked.)
If a key allows downloading a game, it can be used multiple times by different users (unless it is revoked). However, if the key can also be "claimed," and someone has already claimed it, then it becomes unusable for other users. When you generate a key, there's an option to prevent it from being claimed by any user.
A key that allows downloading but cannot be claimed should not add the game to the library, but I've never tested this, so I can't say with 100% certainty that it actually works that way.