A grapefruit spoon is about the size of a teaspoon. It differs from normal spoons in that its end is sharp and serrated. The serrations make it easy to slice and scoop out the sections of a grapefruit from the membrane inside the peel. There is also a grapefruit knife for the purposes of grapefruit dissection, which I've never seen.
The serrations are almost dangerously sharp, so one must be careful when eating with one.
The grapefruit spoon predated the almighty spork by quite a bit (thanks, C-Dawg!). It may have been an inspiration to its design.