The word cabochon comes from the French word "caboche," which means a knob or small dome." A cabochon normally has no facets, although there are exceptions here too. Occasionally, transparent stones such as emeralds, amethyst and garnet, are also sometimes fashioned as cabochons.
Photo credit Tammy Powley of about.com: jewelry making
Many cabochons are round or oval about other shapes are fairly common too, such as navette and marquis shapes. Cabochons can be made of various materials - glass, lucite, gemstones, etc. There are rhinestones which have flat backs too, but they are more commonly referred to as flat backed rhinestones rather than cabochons.
About.com has an interesting article on the process of cutting a stone cabochon that you may want to read if you would like further information on the subject.
Here are some photos of vintage jewelry pieces which use cabochons. All items are available from my store Vintage Jewelry Lane. Click the photos to go to the listing page. Prices range from $9.99 to -31.99.