A style of skiing, also referred to as cross country.
When compared to downhill or alpine skiing, many people thing that nordic skiing is a lot of hard work. While this may be true, there are a lot of more interesting differences to worry about.
For example, your average downhill ski weighs in at about 3-5kg (6 to 10ish pounds for you Imperialists), most light
track nordic skis weigh between 0.5kg and 1.5kg (1lb to 3lbs). This makes moving around on them much easier.
Nordic skiing traditionally occurs on flat or rolling terrain, skinny skis aren't so great for negotiating steep descents.
Nordic ski bindings (where you attach your boot to the ski) traditionally only attach at the toe. This allows you to lift your heel whilst on the ski which makes it easier to walk/run while having sticks attached to your legs.
There are two major techniques used to nordic ski. Firstly, the classic or diagonal stride which is what most people imagine when they think of cross country skiing. It is kind of like walking or running on skis. The second major technique is the skate. This looks basically like skating on rollerblades, except you've also got poles and you get to do it on snow where it doesn't hurt so much when you fall on your arse. Competition skaters maintain speeds around 30km/h for distances up to 20km. Shorter sprint events can attain speeds of 40-50km/h consistantly.