An artillery shell consists of an encased explosive charge with a fuse to allow for detonation. Most modern shells are conical in shape with a contact detonator in the tip. When fired from a cannon, the shell flies point-first toward its target, and when the shell impacts the ground or a hard surface, the contact detonator fires, setting off the explosive charge in the shell.
The earliest cannons used solid projectiles, which made them useful against fortifications but not as powerful against troops on the field. In an attempt to increase the damage potential of cannons, artillerists began drilling into their cannonballs and filling the shot with powder, creating a charge that would blow apart the shell and cause more damage. These earliest weapons required timed fuses, were lit before firing, and could be unpredictable. The line "bombs bursting in air" in the Star-Spangled Banner refers to these explosive cannonballs detonating high above the battlefield.
Modern artillery shells come in a wide variety of configurations. Simple explosive shells rely on the charge turning the outer casing of the shell into shrapnel, while antipersonnel rounds contain extra metal projectiles inside the shell to increase casualties. Armor-piercing rounds are made of denser materials than other kinds of shells to direct more kinetic energy to the target.