Grape shot is a canvas bag packed with small metal balls ("grapes"). This is protected from the gunpowder charge by metal wadding. As the name suggests, the balls fly through the air in a cluster, shot by a gun or a cannon. Grapeshot was much used until it became obsolete during the 19th century. These days it would be called an antipersonnel weapon.
An artillery shell is a hollow ball packed with explosive and a fuse. The fuse is lit by the explosive charge that sends the shell towards its target. It is designed to explode when it reaches its target. Grape shot does not wait. It begins to spread as soon as it leaves the gun or cannon. Grape shot is quite different from a shell or shrapnel rounds.
Shrapnel, while similar to both, is different and more deadly. Invented by a British officer during the Napoleonic Wars, it is a fused explosive shell filled with many small balls or iron/lead bits. When the shell bursts the small bits scatter in a more or less spherical pattern with usually fatal force.
All three of these rounds, grape shot, shell, and shrapnel, are extremely effective against troops, horses and other livestock. They can also destroy wooden structures. Shells have historically been effective as incendiaries against wooden targets, which they ignite from the heat of their explosive blast. Grape shot sends out mini-missiles across a wide pattern. This is best for human targets, but it may be damaging against nearby wooden structures.