Topsoil is the top layer of soil that extends from a few inches to several feet deep, that has an open structure containing enough oxygen necessary for plants and all soil organisms to survive.
In semi-arid and arid regions, the topsoil may only be a few inches deep, and could be mostly sand or fine, compacted soil particles.
Beneath topsoil is subsoil in many regions. In desert areas, topsoil, what there is of it, may overlie hardpan or caliche.
See also:
Caliche - Hard Desert Gardening