It is probably the most durable painting/drawing medium in existence. Paintings created with it require no restoration and many works created as far back as the 16th century still exhibit their original brilliant intensity. Pastel is derived from the french word pastiche, which translates as 'imitate', possibly because pastels were first designed as an alternative to oil. It is primarily composed of pigment ground into a paste mixed with a binder, gum tragacanth, and a minimum amount of clay or kaolin. Unlike oil, none of these compounds cause the pigment to darken, and pastels will not fade with time like watercolors.
Modern pastel painting was popularized in France, invented by a German, Johan Thiele, and first consistently used by an Italian, Prosalba Carriera. Arguably the most famous early pastel artist is Edgar Degas, who is popular for his paintings of dancers which capture a strong sense of motion and light. Pastel use first arose in response to the difficulty of working with oils but quickly found its own unique niche as a medium for artistic expression. It is quick and easy to work with, allowing broad emotive strokes, and requires no drying time.
Sources:
http://www.pastelsandpens.com/aboutpas.htm
http://www.collectorsguide.com/fa/fa071.shtml
http://www.tfaoi.com/newsm1/n1m136.htm