A propulsive charge sends the shell into the air. A mortar shell can be launched thousands of feet, which makes it an indirect fire weapon. That means the people launching the weapon cannot see their target, or it means that the weapon fires at a high trajectory. Under the Oslo peace accords, mortars are banned from Palestinian territory. Although the shells launched by the Palestinians were of low quality and caused little damage, a small high-quality shell is the equivalent of a stick of dynamite and can blow up a car.
Larger shells are several sticks' worth and can destroy a house.
Shell is a generic term for many different types of projectiles. Mortar shells, for example, are metal casings filled with an explosive charge. Shrapnel has come to refer to the pieces of flying metal from a shell casing. Rockets are more sophisticated devices than mortars. Rockets are self-propelled, which gives them greater range than mortars.