Cross Country biking requires much more endurance than downhill and is normally ridden at a slower pace than downhill. Cross Country trail riding goes over many different terrains and hills.
Type of bike
The bikes used in Cross country are much different from downhill bikes. They are made to be as lightweight as possible and most modern XC bikes are made of carbon. The suspension on the bikes is very lacking because it is not as necessary as in downhill. The suspension is normally about 100-120mm of travel and very often they are hardtails. Since they don't go on steep downhills like downhill races the bikes are more "square shaped" and have less aggressive tires and geometry.