Sheriff

At night the sheriff checks one player and learns “Mafia” or “Not Mafia”. The don counts as “Mafia” to him too.

The town's detective

At night: Checks one player and sees: “Mafia” or “Not Mafia”.
Goal: Find the mafia and convince the town.
Tip: Don't rush to reveal yourself: the don looks for the sheriff every night.

In short

At night you wake after the mafia and the don and pick one player.
The answer is “Mafia” or “Not Mafia”. The don also gives “Mafia”.
Only you see the answer, and only for a few seconds: remember it.
You can't check yourself, it's one check a night, and you can't change your mind.

There is one sheriff in the game, on the town's side. At night you wake after the mafia and the don, and before the prostitute and the doctor (if they're in the game). The others sleep and don't see what you do.

Pick one player — and you'll see the answer at once: “Mafia” or “Not Mafia”. It appears next to that player and disappears after a few seconds, so remember it.

What matters about the answer:

the don is “Mafia” for the sheriff too;
“Not Mafia” only means the player is on the town's team: you won't learn their role;
you can't check yourself, and it's one check a night;
whether to say the answer aloud or keep quiet is up to you.

With the automatic host there's limited time to choose, and the room owner sets it. With a live host, the host wakes you and sets the rhythm of the night.

The mafia has the sheriff in their sights too: the don checks players for exactly this role.