Types of Nordic Skiing
The Professional Ski Instructors of America (PSIA) recognizes two Nordic skiing certifications, Nordic track and Nordic downhill. A few PSIA organizations also offer backcountry certification. Though I don't believe PSIA offers a specific certification for this, another type of ski touring is alpine touring or randonnée.
Both classic and skate cross-country skiing are included under the general heading of Nordic track. Classic skiing looks similar to a walking or running motion, but has a distinct glide phase and a pole push. The actual skiing motion is considered a diagonal stride because the opposite arm and leg move forward at the same time. This type of skiing can be done on both groomed trails and terrain that has not been groomed.
Groomed trails can be laid down on any surface that is primarily free of large debris such as trees and rocks. Groomed trails require snow grooming equipment that comes in many variations and price ranges. In the simplest form, some clubs groom their trails by running a snowmobile around a city park a few times to create a circular ski track. This can also be done at a golf course, open space, closed road system and on private land. Of course, the appropriate permission to access the property must be obtained.
Major Nordic centers and ski areas typically use larger pieces of equipment able to lay down a large volume of track in a short period of time. A sample resource list for larger pieces of equipment can be found on xcski.org.
Skate skiing, also known as Nordic freestyle, requires groomed trails in most circumstances. Skating is a type of skiing where the skis maintain a divergent relationship. Viewing the skier from the back, the skis would appear to make a "V" shape. Forward propulsion is achieved with a skating motion, similar to high-speed ice skating. The timing of the pole plant varies depending on the particular poling technique being used, while the skating leg motion remains essentially unchanged.
If athletes are racing or going uphill, more upper-body strength and stamina are needed due to a more aggressive use of the poles for propulsion.
The sport of Nordic downhill includes both parallel and telemark skiing techniques. While parallel skiing techniques more closely resemble those used in alpine skiing, in telemark skiing feet are displaced fore and aft with the entire front foot and ball of the rear foot pressuring the skis. Telemark ("tele") turns are led with the heel flat on the outside ski (the downhill ski at the end of the turn), while the inside (uphill) ski is pulled beneath the skier's body with a flexed knee and raised heel.
Backcountry skiing usually refers to free-heel skiing outside of ski area boundaries and off of groomed trails. Backcountry skiers usually carry loaded packs, and their ski equipment is heavier to accommodate the load as well as the rough terrain. People that participate in backcountry need good fitness and skiing technique, and they need to be well educated in orienteering, mountaineering and first aid.
More: Backcountry Bliss