Cross-country (XC) mountain biking describes most styles of mountain biking. In recent years, there has been a push towards diversifying this group into other genres of MTB, which include all-mountain and trail bikes. These genres have been created to help differentiate the bikes' intended purposes and have helped specialize the diverse nature of XC riding.
Cross-country riding, for the purposes of this article, is any situation where a rider is able to ride the entire trail without having advanced technical skills -- like needing to cross a 6-foot gap to stay on the course -- and is able to start and stop from the same spot (no shuttle or lift is needed). Point-to-point riding is a part of XC riding, but you have the capability of going in the reverse direction without outside assistance. This leaves a broad spectrum of what's considered XC, which is why so many people have embraced this style of riding as it encompasses so many different skill levels.