Class III slots are probably the kind you're more familiar with. Also referred to as "Vegas slots", class III games employ an RNG that randomly selects random numbers corresponding to predetermined payouts. You play class III slots by placing a bet and telling the game to take an instantaneous snapshot of the current RNG random number and display the payout it represents. Class III games run independently of each other. It's just you and the RNG.
Class II slots are connected to a single serve that determines outcomes in a way similar to Bingo where there is only one winner per outcome. In this case, you're playing against other players for winning outcomes each spin.