Skiing (technically snow skiing) is the activity of gliding over snow using skis (originally wooden planks, now usually made from fiberglass or related composites) strapped to the feet with ski bindings. Originally used primarily for transportation, snow skiing evolved into a popular recreational and competitive activity during the 20th century. The sport consists of many variations, each of which fits into one of the major categories of alpine skiing, Nordic skiing, or telemark skiing.

In Europe, pistes are classified by a colour-coded system, although shapes are not always used (sometimes all ratings are circles). The ratings are:

Green - (Spain, France,Scandinavia) Learning or "Beginner" slopes. These are usually not marked trails, but tend to be large open, gently sloping areas at the base of the ski area.

Blue - An easy trail, similar to the North American Green Circle, and are almost always groomed, or on so shallow a slope as not to need it.

Red - An intermediate slope. Steeper, or narrower than a blue slope, these are usually groomed, unless the narrowness of the trail prohibits it.

Black - An expert slope. Steep, may or may not be groomed, or may be groomed for moguls. "Black" can be a very wide classification, ranging from a slope marginally more difficult than a "Red" to very steep avalanche chutes like the infamous Couloirs of Courchevel. France tends to have a higher limit between red and black.

Orange - (Austria, Switzerland, certain other areas) Extremely difficult.

Double-black - (Scandinavia) Extremely difficult.

Alpine slope classification in Europe is less rigidly tied to slope angle than in North America. A lower angle slope may be classified as more difficult than a steeper slope if, for instance, it is narrower and/or requires better skiing ability to carry speed through flatter sections while controlling speed through sharp hairpin turns, off-camber slope angles or exposed rock.