Mountain bikes are specifically designed for biking on dirt trails and other unpaved surfaces. Their tires are wider than those used for road bikes, and they have larger treads for better traction. The larger wheels are better able to scale obstacles, but larger tires also cause the bike to be less maneuverable.
The first piece of equipment required for distance touring is a touring bicycle. While many people choose mountain or all-terrain bikes, this type of bike is heavier, less comfortable, and slower than a traditional touring bike. Unless most of your trip will be off-road, go with a good touring bike that fits you correctly instead.
USA Cycling is the national governing body for the sport of cycling in the United States and oversees the disciplines of road, track, mountain bike, cyclocross, and BMX.