Grape shot is a hollow ball, packed with small "grape" sized metal balls which can be fired by a gun or a cannon. The hollow ball is fired as a single round that breaks apart upon leaving the muzzle of a cannon. Grapeshot was much used until became obsolete during the 19th century.
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 from their fragments. Shells have historically been particularly 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 people targets, but it may be quite damaging against nearby wooden structures.