A relief is a sculptured artwork where a modelled form is raised, or in sunken-relief lowered, from a plane from which the main elements of the composition project (or sink), says Wikipedia.
Reliefs are common throughout the world, for example on the walls of monumental buildings. The frieze in the classical Corinthian order is often enriched with bas-relief (low relief). Alto-relievo (high-relief) may be seen in the pediments of classical temples, e.g., the Parthenon.
A bas-relief ("low relief", French pronunciation: [baʁəljɛf], from the Italian basso rilievo) or low relief is the quality of an projecting image where the overall depth is shallow. The background is very compressed or completely flat, as on most coins, on which all images are in low-relief.
Lorenzo Ghiberti's gilded bronze "Doors of Paradise", Baptistery, Florence combine high relief main figures with backgrounds mostly in low relief.
Bas-relief is very suitable for scenes with many figures and other elements such as a landscape or architectural background. A bas-relief may use any medium or technique of sculpture, but stone carving and metal casting are the traditional ones. If more than 50% of most rounded or cylindrical elements such as heads and legs project from the background, a sculpture is usually considered to be "alto rilievo" or "high relief", although the degree of relief within both types may vary across a composition, with prominent features such as faces in higher relief.
Illustration above from Darius the Great's audience hall in Persia, today's Iran. Darius I was both builder of the vast Persian Empire and an enlightened ruler.