The Rubber Band Gun
The classic rubber band gun. They have been around for a long time an can be made from almost anything. Basically they are all about releasing a rubber band by a trigger, down a stick. You can make them as pretty (or as ugly) as you want but other than looks they are differentiated by their release mechanisms.
Basic - The Peg
Other peg method, not so common but most say better, is to have the peg when pressed push the band off a nail. You drill a hole through the end of the peg and put a nail through it so that it sticks out when the peg is not pressed and doesn't when you press the peg. To fire it, the rubber band sits over the nail and when you press it the 'floor', the rubber band is against, rises and the band is released.
More Complex - The Cog Wheel
Rifles don't normally have different mechanism, just different shapes. Quite often they boast longer distances and better accuracies though.