It is actually a form of calcium carbonate. The word is Spanish, but comes from the Latin word calx, which means limestone.
It can be a few centimeters thick or many meters thick. It occurs all over the world in deserts and semi-arid areas. The Kalihari and Sahara Deserts have large deposits of caliche, as does the Arabian Peninsula, western and central Australia, and the southwestern deserts and high-plains regions of the U.S.