A Cabochon is a gemstone which has been shaped and polished as opposed to faceted. Cabochons or Cabs are generally circular or oval but may be cut into different shapes with a flat reverse side. Cabochon was the default method of preparing gemstones before gemstone cutting was developed.
Cabochon are usually used on softer species, such as Opal or Moonstone (Mohs Scale of Hardness), opaque gems or gems of a lower grade. So Cabochons are often act like a window, allowing one to see the gems natural details inside and display other visual effects such as play of color in it. Cabochons are more likely to have natural inclusions and markings.
Cabochon can be set in different ways but one of the most common ways is with the bezel setting, so a bezel setting is a wall of metal that surrounds the outsides the stone normally sterling silver, you then push that metal over the curve or dome of the stone.