The mountain bike is a specialized bike for rough terrain off-road. This sport covers many activities, whether in competition or simply for leisure.
DH or downhill consists of riding a very technical course with a negative gradient and numerous obstacles where riding and speed management skills are essential.
Cross-country is the endurance discipline of mountain biking, it is practiced on more varied terrain. This discipline requires not only real technical riding skills, but also a good physical condition to manage your effort.
There are other disciplines in mountain biking such as enduro or trial which are less well known, but which require various skills such as endurance or notions of balance and control of the bike.