The Janitor is an anti-Town role who "cleans up the body" after its kill, thus preventing players from learning anything about the player when they flip. At daybreak, it is simply said that the player died with no information about their role or their alignment.
Usually Janitor is X-Shot so as to not deny the Town too much information. In this case, the Janitor must choose whether or not to clean up the kill.
Usually, but not always, Janitors will learn the role and alignment of the players they kill even if they clean up the body. Whether this information is revealed to the player or to the team at large is also up for debate.
Use and Power
Most of the time, Janitors are scum, so the people whose flips are obscured are usually Town. Thus, the use of a Janitor revolves around obscuring whether the killed player had a power role. Thus, Janitor is best used when killing players who have not claimed.
As for its power, it's probably overrated.