As mentioned elsewhere in this site, mountain bike wheels tend to be larger and wider than the wheels on road bikes. In fact, measuring bicycle wheel sizes is somewhat imprecise. Nevertheless, mountain bike wheelsets come in a variety of diameters and styles, usually wider than their road-based counterparts. There also exist a number of tread patterns designed to meet different purposes.
Wheelsets range from standard mountain bike rims to the less wide ones which are popular with freeride and downhill bicycle riders. Heavier has usually been thought to be better for downhill wheelsets, but continued improvements in technology have allowed designers to continually lighten the weight of wheels without losing durability.
Some wheelsets are specifically designed for winter use, and these are the widest wheel widths out there. They are meant to be ridden in the mountains during snowy conditions. Other wheelsets and corresponding tires are designed to be more applicable all-around, and are less specialized. The type of wheelset that is best for you will depend on what kind of riding you are looking to do.