Skiing is either sportive or recreational activity using skis for sliding over snow. Skis are used with special boots connected to them with a binding. There are two different types of skiing: downhill skiing and cross country skiing.

Downhill skiing 
Downhill skiing is a sport. Skiers ski down a trail (also called a "run") on the side of a mountain or hill. Each trail is marked with a sign that shows how difficult that particular run will be. Three different colors are used to rate the difficulty of the trail. They are: green, blue, and black. Green trails are easiest, blue trails are more difficult, and black trails are rated most difficult. The colors which represent a trail will also be seen in a shape.  

The most common ratings for trails are:
 Green circles are the easiest trail. They have a very shallow slope, and the runs are very smooth. They are very wide, and are easily accessed. They are used by beginner skiers to learn.
 Blue square trails are more difficult. The trail will go down a steeper part of the mountain. Sometimes, the trails are narrower, and they may be bumpy. These are advised for beginner skiers wishing to improve.
 Red trails are more difficult. The trail is often longer and steeper and sometimes are less smooth than other runs. These are advised for more advanced skiers.  Red trails are rare. Different ski resorts call them different things, such as double blue square trails.
 Black diamonds are very difficult trails on the mountain. They are often covered in ice or large bumps designed often to challenge skiers. These are advised for very good skiers only.
 Double black diamonds are for expert skiers only. They may have unmarked obstacles and may be very thin runs. They sometimes have to be accessed by hiking up to them, being too steep, rocky, or unused to put in a lift.
 Rarely, there are extreme terrain trails, represented by two black diamonds, one with an E and the other with an X in the center.  These are almost always accessed by hiking, and are for expert, extremely experienced skiers ONLY.
 There are also terrain parks for skiers who want to person stunts and tricks. It consist of tubes, flip courses, and obstacles. 

Trail ratings are often relative to the rest of the terrain at the ski resort- for example, the difficulty of a black diamond trail at a small ski area may be a rated as a blue square at a bigger resort with harder trails.

Cross country skiing 
Cross country skiing is a sport of endurance. Skiers ski anywhere where there is snow. This is usually through a scenic forest or town.

Alpine skiing 
Alpine skiing is a contest of speed. Racers go through the course with red and blue gates and go down to the goal as fast as they can.  There are four disciplines in alpine skiing: downhill, super giant slalom, giant slalom and slalom.

Ski gate

A ski gate is two sticks on a skiing race course that a skier needs to pass between. Ski gates are used in alpine skiing in downhill, Slalom, Giant Slalom and Super-G races. Downhill races have not many ski gates so skiers can be safe when they are going very fast. If a skier does not go between a ski gate, the judges say they are removed from the race. First in the 1980s and now a lot of the time, ski gates have hinges near the snow. Some ski gates have flags at the top of the stick.

References

Other websites 

 Sports: Winter Sports: Skiing  Directory of skiing related links
 National Ski Patrol 
 Ski tips Tips for getting in shape and improving your skiing.
 Is Global Warming Affecting Skiing How Global Warming is Ruining Skiing

 
Winter Olympic sports