The names will change based on the setting, but the basics of each role will remain the same. Not all roles will necessarliy be used in each game.
Villagers/Townsfolk: This is a game of an uniformed majority against an informed minority; and most players will take on the roles of villagers, courtiers, or other innocent parties. Their goal is simple -- to survive -- and they have just one weapon at their disposal: their votes. Each day, they vote to lynch or otherwise remove one other player. Never underestimate the power of a single vote.
Seer/Historian: There's always someone in every group who seems to know everything that's going on. This player can choose one other player during each night and request to learn that player's role. Knowledge is power, but sometimes it isn't easy to collect it quickly enough.
Guardian/Bodyguard: This person protects the weak in a corrupt world. Each night this player will be able to protect one other player from attack by evildoers. It isn't easy to know who to shield, but sometimes taking a chance pays off.
Priest/Judge: Once each game, this player may stop a lynch/removal and make a different sort of test as to the guilt or innocence of the person about to be lynched. If innocent, this intervention saves that person; if not, the lynch proceeds on schedule. The ability cannot be used again in either case.
Alchemist/Mad Scientist: Once each game, during the night, this player may poison or otherwise remove one player of their choice from the game. This role must often act on imperfect information, but this unexpected strike can easily change the balance of a game.
Vampire Sire/Alpha Wolf: This villain can use their ability, once per game, during the night phase, to lure another player over to the dark side. Unless shielded by the Guardian -- in which case the ability is wasted -- the targeted player loses any other special abilities and works wholeheartedly with the villains from that night on.
Werewolves/Mafia: Or whatever else you care to call your villains. They know who they can trust, but they're sadly outnumbered and must work in secret, under cover of darkness. Sooner or later, they'll give themselves away somehow. But will it be too late? These rascally characters vote among themselves on a player to remove from the game each night. If successful, that player's role will be revealed, and the bad guys will be one step closer to victory.