There are three types of sculptures out there: high relief, low relief, and intaglio.

High relief: High relief sculptures stand very far out from the background. With these type of sculptures the forms and figures volume are very strongly projected. They usually extend at least half their depth. It is sometimes called alto-relievo.  This is a famous example of a high relief sculpture that I also really like.

Low Relief: These low relief sculptures are best known as bas-refielf. These kinds are shallow. The sculptures are barely more prominent than the surface. Their created by carving away at the material. Sometimes to do it people start with a smooth surface and then add the material on top. This example is made by Lorenzo Ghiberti.

Sunken Relief: Sunken relief sculptures are also known as intaglio. They are carved although the backgrounds aren't as low. The points in the highest relief are always level with the original surface. To put it in short usually the background is higher than the picture itself. You mainly see these types of sculptures as hieroglyphs. This is an example.