Bas-relief is a type of sculpture that has less depth to the faces and figures than they actually have, when measured proportionately (to scale). This technique retains the natural contours of the figures, and allows the work to be viewed from many angles without distortion of the figures themselves.
There is a continuum of the bas-relief technique into the next category, alto-relievo, or high relief. That technique combines the rounded figures with significantly deeper backgrounds. Instead of backgrounds being between a fraction of an inch to few inches deep as they are in bas-relief, they may be a foot to several feet deep in alto-relievo.