In thinking about the type of cycling that you want to do, if you plan to ride “off road” a lot, like using your bike to traverse the woods or zip through the desert on sandy or rocky trails, a mountain bike is probably what you want. Mountain bikes are designed for riding under more rugged conditions, and typically:
have a stouter, more upright frame
offer higher clearance to get over rocks, logs and through ruts, etc.
can take a lot of stress and abuse and still allow the rider to comfortably negotiate rugged terrain and go over or through obstacles that he or she may encounter on the trail.
A mountain bike usually has wide knobby tires that offer more substantial grip and traction on a variety of surfaces, including gravel, dirt, rock and sand. Tire pressure on mountain bikes is less than on road bikes, due to their greater volume and the better traction offered by a softer tire. The rims and spokes on mountain bike wheels are stronger and more durable, again to handle the rougher riding that true mountain biking entails.
Most entry-level mountain bike frames are steel (also called “cro-moly”), due to the strength and durability the material offers and the (relatively) low price of steel. Disadvantages of steel are its weight and that it can be prone to rusting.
As the materials get more advanced, the price increases. Next on the ladder is aluminum, which is light and rust-proof and relatively strong, but not immune to breakage over time when subjected to repeated stress. I’m not a fan of aluminum in mountain bike frames, simply due to the rough nature of riding, which constantly subjects a mountain bike frame to significant stress, particularly when a heavier rider is involved.Carbon-fiber frames, the next higher level of material, are similar to aluminum in being light, rust-proof and very strong, but also prone to breakage, but when they go it happens suddenly, and usually at the worst possible time.
Top-of-the-line mountain bike frames are made from titanium, which is super light and incredibly strong. In contrast to aluminum and carbon fiber, both of which can eventually fail over time, a titanium frame on a mountain bike will be a great choice for large riders because of its ability to handle larger loads without becoming fatigued. There’s always a trade-off, though, and the downside is that titanium is very expensive. Because of its cost, these bikes with titanium frames typically fall beyond the range of all but the most serious or competitive cyclists.
Mountain bikes have a wide range of gearing to allow them to handle a broad range of terrain. With low gears that go well below that of most road bikes, riders are more easily able to conquer some wicked steep hills. On the high end of the gear range, mountain bikes typically are typically not so tall in the gearing as what you’d find on a road bike. Rarely is there the need for wide-open, blazing speed such as you’d have on a road bike, and the bike’s over-sized, knobby tires are not really conducive for going lightning fast anyway.
Typically a mountain bike will have either two or three chain rings in the front as part of the crank assembly, again smaller than what you’d find on a road bike, along with eight or nine gears in the cassette on the rear wheel, many times featuring one bodiaciously-sized gear called a granny gear to help with the particularly steep climbs. This combination allows for anywhere from 16 to 27 possible gear combinations, a range that accounts for virtually every type of terrain that a mountain bike will encounter.
Basic mountain bikes come equipped with platform pedals. This is useful if you’re the type of rider who frequently puts your feet down. Other more advanced riders may prefer to use toe clips or even clipless pedals that allow the rider to secure his or her cleated shoes to the pedals, but people have different levels of comfort when it comes to being fully attached to a mountain bike given the varieties of terrain encountered and the frequent need to drop ones feet to the ground.
For a mountain bike, you can buy a cyclocomputer, frame pump, tool bag, water bottle and cage. Also, given the rugged nature of the riding and the frequent distance from “civilization” that mountain biking trails go, many mountain bikers carry tools kits outfitted with a good many more tools and replacement parts than their roadie counterparts need to bring along.
Be sure you will be doing plenty of off-roading before you buy a mountain bike. If you are just going to be riding in town or on paved or hard-packed smooth, flat trails, there are better choices of bikes that will be more comfortable and serve you better, such as hybrids or cruisers.
Also, beware of cheap, heavy mountain bikes sold by mass-retailers. Though they may offer snazzy-looking front and rear shock set-ups, generally these add a lot of weight to the frame, and being made from cheap components, will not last very long under any type of rugged riding conditions.