(Setup) sweeper: Pokemon that can "sweep" through the opposing team. They usually have boosting moves and are mostly used on (hyper) offense. Examples include Quiver Dance Volcarona and Dragon Dance Roaring Moon.What exactly is the difference between a sweeper/cleaner/wallbreaker? Seems like a lot of different terminology to refer to offensive mons
Cleaner: Pokemon that can "clean up" the opposing team after holes in the team have been made. They are usually fast, possibly thanks to Choice Scarf or Booster Energy, and/or have priority moves and are mostly used on (bulky) offense and balance, in tandem with a wallbreaker. Examples include Choice Scarf Enamorus and Booster Energy Iron Valiant.
Wallbreaker: Pokemon that can dish out strong attacks to "break walls" and make holes in the opposing team. They usually have high attack or special attack, possibly thanks to Choice Band or Choice Specs, and are mostly used on (bulky) offense and balance, in tandem with a cleaner. Examples include Choice Specs Kyurem and Choice Specs Walking Wake.
Keep in mind that many Pokemon and even certain sets, like Kingambit with Swords Dance and Sucker Punch, can fulfill multiple of these roles, depending on the team and the flow of the game.