There is currently only one mode that players can que up for matchmaking
However in Private Games you can host Classic Mode that simplifies the game. This is most often used when veteran players guide "Newbies."
Casual Mode is a simplified version of Classic Mode. Some classes will not appear, and the Cult faction will not spawn, meaning the Unseen are guaranteed to do so. Additionally, any class that would have Royal Blood that spawns will not have it.
During Beta testing, there was a mode named Aftermath that featured voice chat, was played with 5-8 players, and had a Werewolf-style gameplay rather than a Mafia-style one. However, this experimental mode ended up being retired due to not being considered very fun.