A Deflector is a role that takes all actions that target one player at Night, and redirects them so that they target a second player instead.
A Deflector may or may not be able to target themselves.
Until 2013, this role was often named Redirector instead. Nowadays, that name is used for a night action which, instead of redirecting all Night actions that targeted a specific player, redirects all Night actions performed by a specific player (previously, both roles were called Redirector).
A Bus Driver is a Deflector who swaps all actions performed on two players -- all actions that target the first player now target the second, and all actions that target the second now target the first. This swapping of two targets is referred to as "driving".
Use and Power
In the majority of cases, players are not informed that their Night actions were redirected. There is no consensus about whether an investigative role should be made aware of this redirection via their report or not; see Dealing with Scum Bus Drives.
Note that a Deflector can function as a very powerful protective role, as the player targeted essentially becomes untargetable by Night actions. A Town Deflector can use their power on a Cop and make them immune to both kills and roleblocks, protecting them even better than a Doctor could.