The Bus Driver is a role that can choose to switch two players each night. Thus, any night action performed on a switched player will affect the other player targeted by the Bus Driver. The Bus Driver's night choices are only valid for that same night.
For example, suppose a Bus Driver switches John and Jack. That same night, a roleblocker tries to block John and a Goon tries to kill Jack. In the end, Jack will be roleblocked and John will be killed.
The verb form of what a Bus Driver does is "driving", as "bussing" is a different concept entirely.
Bus Driver has been seen as each alignment and historically is a pro-Town role; however, some moderators consider Bus Driver to be a Negative Utility role in Town hands, since it is more likely to thwart or confuse power roles than help. The pro-Town intent of this role is to attempt to redirect the scum's kill onto a better target.
The Bus Driver role is a special case of a Deflector, though it is interesting to note that the Bus Driver was created first.
Many, but not all, moderators do not allow Bus Drivers to self-target (i.e. choose themselves as one of the people they swap).
There is no consensus as to whether investigative roles (e.g. Cops) that have their target changed by a Bus Driver are told that their target changed.
Use and Power
As a pro-Town power role, Bus Driver is similar to a Doctor in that it can prevent a player from being killed if the Bus Driver targets them each Night. However, it does not stop the kill on its own, so it is not as much of a disadvantage to scum if used in this way. With that said, it is far more likely that the Bus Driver will hurt the other power roles as their actions get unexpectedly redirected by the Bus Driver's choices. Thus, it is not considered a great power role.
If using a pro-Town Bus Driver, it is important to consider how the Bus Driver can cause broken combinations. In particular, including a Bulletproof in the game can allow a Bus Driver to functionally become a Doctor.
As an anti-Town power role, Bus Driver is rather potent. By redirecting up to two power roles per Night, they can neutralize a lot of Town power at once - if not manipulate their actions so that they actually help scum. An anti-Town Bus Driver that can self-target is very nearly immune to all Night actions.
As long as a Bus Driver is in the game, there is a chance for scum to target one of their own. Thus, it tends to extend LyLo and MyLo, for example, going into the night with a Town Bus Driver, another Town member, and two Mafia can still be won by the town if the Bus Driver targets the right people, whereas normally it would be considered a win for Mafia.