A powerful type of particle weapon
Proton Cannons fire bolts of Protons, either through EM forces, or Ionic Clouds. The Protons slam into and destabilize some of the molecular/atomic bonds in target, causing low level nuclear fission i.e. the target explodes.
Because of the small nature of a Proton (It's a fundamental particle) a great deal of energy is used moving its about safely and firing it.
Proton Cannons are considered illegal by most advance civilizations that can create them. They are also not used often as they are energy intensive.