Most are located along the edges of crustal plates, especially it seems the Pacific plate. A few are located over 'hot spots' (example: Hawaii's Kilauea volcano). Now many on the edges are either near where the seafloor is spreading or where one plate is going under another (subducting). A full 1/4 of the active volcanoes on the planet are located in Indonesia (an area where 3 plates are involved). However the entire Pacific ocean has volcanoes scattered around it.