This type of gun generally is inexpensive and readily available in most of the place due to simplicity of the mechanisms. These guns are the entry-level to the airsoft game.
Mechanisms of the gun is using a coil spring to store elastic potential energy by compress the coil spring. The energy released upon the trigger-pull which will pressurizes the air in the pump cylinder rapidly and blow the pellets out from the gun barrel. Due to the mechanisms of the system that need to recompress the spring before each shot, these kind of guns are incapable for semi automatic or automatic firing.
Normally, these guns are less powerful as compare with gas blow back gun (GBB) because GBB gun system can produce higher pressure thus increase the velocity of the pellets. For those stronger spring which can store higher elastic potential energy can easily shoot a pellet up to 200m/s which can be deadly too.