Cross-country skiing is a diverse sport. While true cross-country techniques are used primarily for transportation and racing across varied terrain, the term has come to include some types of backcountry skiing, and some gear has been designed to be useful in both environments. Typical cross-country skis are very long and narrow, normally ranging at around 6 feet long, and have light bindings that allow the skier's heel to flex above the ski. They are designed to spread out a skier's weight and reduce friction when moving across relatively level snow.
The telemark style was developed for negotiating downhill runs on cross-country skis. Like most cross-country designs, telemark bindings leave the heel free at all times, making them efficient on level terrain. However, they are typically wider and heavier than classic cross-country styles. Telemark skis also come equipped with reinforced edges for cornering on downhill runs. This type of ski can be used for both backcountry transportation and for skiing ungroomed, off-piste slopes. Many skiers find the downhill technique efficient enough to bring the skis to groomed hills, as well.
Randonnee, or alpine touring skis, favor the downhill more than other types of cross-country skis. The distinguishing feature of randonnee equipment is the ability to lock down the skier's heels. Consequently a skier can negotiate cross-country routes and even steep inclines with the classic free heel. Then the skier can lock the bindings and ski downhill runs with techniques similar to recreational downhill skiing. Many skiers new to cross-country and backcountry skiing favor randonnee equipment for this reason.
"Backcountry" refers to larger, heavier styles of cross-country skis that can be used in true backcountry environments. Also referred to as Nordic designs, these skis excel in ungroomed snow where the typical lightweight cross-country equipment would be inefficient. The bindings on these skis are very similar to telemark designs, including a free heel, and downhill skiing requires a similar technique. However, Nordic skis cannot negotiate the same demanding slopes as telemark equipment.