A Cabochon is a gemstone that 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 is 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 acted as a window, allowing one to see the gem's natural details inside and display other visual effects such as a 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 outside of the stone normally sterling silver, you then push that metal over the curve or dome of the stone.