From SA Mafia Wiki
A setup is considered Open if all the players go into the game knowing all the possible roles and actions for the game. A list of popular open setups can be found here. Note that open setups are at a greater risk of the town attempting to break the game, since they can figure out all the possibilities and prepare for them appropriately.
A setup is considered Closed if the players enter in knowing nothing about the game. Most setups on SA are closed.
If a setup is mostly open, that is, the Mod reveals the majority of the setup but leaves certain parts out, then the setup is considered Semi-Open.