In its most basic form, an ice cream scoop is a big, tough spoon, with a little more 'depth' to it, and a broad, flat tip for scooping. Variants include conical scoops, spade-like scoops, spatula scoops, etc.
Ice cream scoops have three main advantages over just using a regular spoon. The first is that they are much tougher, so they aren't prone to bending the way a spoon is. The second is that the handles are usually much bigger and rounder than a spoon, so you can get more grip, and aren't likely to cause yourself pain the way a flat-handled spoon could. The third is the way the shape of a round scoop lends itself to creating nice neat balls of ice cream, kind of like a melon baller.
If you like your ice cream served very cold and still reasonably hard, but find it too hard to scoop that way, a good way to deal with it is to heat up the scoop under hot water. This way it will cut through your icecream like a hot knife through butter (or like a hot ice cream scoop through ice cream for you geniuses out there). Most scoops are solid metal, which works fine for this, but some have a hollow core filled with liquid, so they retain the heat from the hot water bath a little better.
Another feature of some scoops is a spring loaded metal strip - operated with a thumb-lever - which slides around the inside of the 'bowl' of the scoop, to make it easier to liberate the ice cream from the scoop. This is especially useful when trying to get the ice cream onto a cone without breaking the delicate waffle material. Some also operate in a slightly different way, having a spring loaded lever that pushes the ice cream out of the scoop from the back.
Scoops designed for commercial use sometimes have a number printed on them. This number refers to the number of level scoops per quart of ice cream.
Props to yclept for inspiration and input.