Mountain bike tires come in an array of shapes and sizes, and choosing the right ones for your purposes can be difficult. The first thing you should do when choosing tires for your bike is to decide what kind of riding you will be doing and in what terrain you will be riding. Tires that perform well in mud may not perform well in dry, dusty conditions, and vice versa. If you are riding your mountain bike primarily on the road, you would need to choose different tires altogether. So step one is deciding what terrain you will be riding most often.
Mountain bikes have become multipurpose in recent years, and therefore cyclists may find themselves riding on pavement more often than any other terrain. If that is the case with your riding, the mountain bike tires you should choose are slicks or semi-slicks. Slick tires have no tread on them and thus have low rolling resistance. Semi-slick tires have little or no tread in the center of the tire but have knobby teeth on the sides, so if you are riding both on and off road, the semi-slick tires afford you low rolling resistance but also traction while cornering.
For those muddy days, your tires must have tread that is spread out enough to shed mud. If your tires become caked with mud, they lose their grip and could cause you to slip over rocks and roots. Good mud mountain bike tires have wide channels that allow mud to flow through them instead of building up and sticking in narrower grooves. A rear tire with wide, horizontal treads down the center and spread-apart knobs on the sides would be a good choice.
Dry, dusty conditions require different tires altogether. Since shedding mud is not important, the knobs on the tire can be closer together and deeper to provide traction in loose terrain. If you are riding slickrock or need your tires to grip in tight corners, consider super-tacky tires which feature a softer compound of rubber. These tires tend to wear out quickly, but in race situations or rides in which grip is vital, super-tacky or soft-compound tires will work wonders.
One important consideration you should think about before buying any mountain bike tires is the type of wheel system you have. If your wheels feature tubeless rims, you should choose a tubeless-specific tire; these can be more expensive, but you will save money on tubes and you can run lower tire pressure for greater traction. Tubeless rims require tubeless-specific tires to connect correctly and seal air leaks; they can also be used with a tube if you get a flat. Non-tubeless tires will not work correctly with tubeless systems; while they will mount fine and inflate with a tube on tubeless rims, you will find it far more difficult or even impossible to inflate the non-tubeless tire without a tube.