Bikes have come a long way over history and the off road mountain bikes we know of today did not become popular until the 1980's.  Mountain bikes are built different than road bikes in the sense they have to survive completely different terrain and abuse.  Most modern mountain bikes have serious components like front and rear suspension, disc brakes and either 26" or 29" tires to take riders over the roughest terrain with ease.  Trailriding mountain biking today is the most popular and consists of navigating singletrack trails that can contain any type of obstacles from steep hills, rocks, logs, cactus, and more to give the rider a challenge.  Other types are downhill, cross country, free ride, street riding, and dirt jumping.  Whatever type you prefer you will find many options of Mountain Bike Adventures here on AdventuresOnly.com.