Now we know you know this one. Riding down hills at high speed? Correct! Downhill is specifically about using DH bikes. The frame geometry of these bikes lends itself to descending and definitely not hill climbing. Usually the biker is dropped at the top of the descent and off they go. Typically this type of event is used at off-season ski resorts. The riders are carried to the top using the ski lifts. The course includes huge drop-off's of tens of feet and extreme inclines. Full body armour is worn with full face helmets.
When most people hear the word term mountain biking they think of downhill. Don't forget half the fun in mountain biking is uphill.