Few rivalries are more heated than the antagonism that exists between cross-country and downhill skiers. Cross-country skiers extol the tranquility that comes from taking an extended trek through the woods. Downhill skiers celebrate the exhilaration and, yes, danger of racing down the side of a mountain. For a pure cardiovascular workout, nothing beats cross-country skiing. Furthermore, this low-impact exercise works more muscles than any other activity.
Classical style uses tracks groomed into the snow.
Skating is more strenuous, requiring the skier angle one ski out in a skating motion while moving the body over the other ski. It's possible to learn cross-country skiing as you go, but for maximum physical benefits, take a few lessons. Cross-country skiing done properly requires high endurance, so it makes good sense to complement the activity with other activities such as running and aerobics. Cross-country simulators, available in a variety of styles and prices, are also useful, but doing well on a simulator doesn't translate to proficiency on the snow. Outdoor skiing requires a longer kicking motion than a machine can be expected to reproduce.