Off-road is generally considered riding on dirt paths with limited vertical climbs and descents. Cross-country is both steep climbs and bumpy descents. Downhilling is long, fast difficult descents. Downhilling is relatively new and being offered during the summer at ski resorts. A chair-lift type service takes you up to the top of the mountain and you take the long wild ride down.
Weight is less a factor in descending as it is in climbing.
The downhilling bike is beefed up with the best suspension systems, disc braking system, an indestructible frame, and only 8 speeds. Cross-country bikes are the most popular. An intermediate level bike will be lightweight, with 24 speeds and a front shock. The more advanced equipment includes a rear suspension and v-brakes, which offer more stopping power. It's important to equip your bike with a quick gear shifting system.