Cross-Country (or X-Country, or XC) is the most popular form of endurance MTBing. It can take the form of an outing or else a race on all sorts of different tracks, sometimes with sections on roads too.
Cross-country MTBing and the bikes used are the closest to recreational and trekking cycling, even if a recreational or a trekking bike will not have exactly the same characteristics as a competition cross-country MTB.
The fact that MTBing is now an Olympic sport has profoundly altered the discipline. The distances and circuits are increasingly shorter with ever greater variety and technical demands.