Mountain bike gloves come in many different styles and varieties. There are a few main types, though: fingerless gloves, full-finger gloves, and cold weather gloves.
During the summer months many cross country riders use fingerless gloves, as they are much cooler and the manual dexterity is much better than in a full-finger glove. However, having the glove padding on the palms of the hands still provides substantial protection from the handlebars, and from the ground in the event of a crash.
Many all mountain, enduro, freeride, and downhill mountain bikers utilize full-finger gloves. Full finger gloves provide more protection in the event of a crash and more grip on the brake levers. Full-fingered gloves for downhill mountain bikers often feature additional protection built into the backs of the fingers and hands in the event of a stray tree or a crash.
Cold weather gloves are also full-fingered, but while other full-fingered gloves often feature advanced ventilation for cooling, cold weather gloves are designed with insulation and protection against the wind in order to keep your hands warm while mountain biking in the fall, winter, and spring.