Mountain bike wheels come in a variety of styles and sizes, but generally speaking they are larger and wider than the wheels on standard bikes. The most common tire size for mountain bikes is 26 inches. Some models also offer wheels ranging from 24 or 29 inches. To make best use of these wheels, tubeless tires with deeper tread are becoming increasingly common.
The larger tires emphasize traction over acceleration, as do the wheels themselves. These wheels would not be as good for racing on the street, but they are much better for climbing steep hills and crossing adverse terrain. The larger wheels are also more easily able to roll over obstacles due to the decrease in approach angle they provide.
Some mountain bikers might also be interested in BMX wheels, which are usually 20 inches in diameter. These are smaller, which makes them stronger and more capable of withstanding landing jumps and stunts. They also accelerate more quickly.