Glissading is the act of descending a steep snow- or scree-covered slope via a controlled slide on one's feet or buttocks. It is an alternative to other descent methods such as plunge stepping, and may be used to expedite a descent, or simply for the thrill. Glissading involves higher risks of injuries than other forms of descending.  
There are three primary methods of glissading:
This is the easiest type of glissade and generally provides the greatest amount of stability. It is also less tiring than a standing or crouching glissade in softer snow. To perform a sitting glissade one sits down and slides on the slope usually holding on to an ice axe in a self-arrest position, especially when the run-out of the slope is in question.
The major drawbacks to the sitting glissade are that one's outer layers get wet, and that there is less control than in a standing glissade.
The standing glissade is often the preferred method if the person glissading is skilled in doing so and snow conditions allow. In this glissading position one has a better view of route hazards, and increased maneuverability over a sitting glissade. In addition a standing glissade cuts down the wet and abrasive forces of the sitting glissade. The standing glissade is best performed over firm snow with a soft top layer.
The crouching glissade is similar to the standing method except the climber sits back and drags the spike of their ice axe (held in self-arrest grip) in the snow. The method is slower but more controlled than the standing glissade. A disadvantage to this technique is the tiring of the legs.