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. After launch, a rocket's combustion products keep propelling it after it has left the tube. A small rocket can go a couple of miles, and a large one can travel 20 miles. The damage done by a single small rocket is equivalent to the damage done by a small mortar shell. A single large rocket inflicts damage equivalent to that of a large mortar shell.