What is print-on-demand (POD) publishing? What are the advantages?

Print-on-demand publishing, or POD, is a method of book printing that uses digital technology to make it easy and cost-effective to produce books as they are ordered, either one at a time or in large numbers. Instead of a more traditional mode of printing, which only prints a large number at one time, POD is an "as needed" printing system, so there are no wasted books and no need to store them. This also means that the book can be available for an extended period of time, instead of the book being "out of print" after a print run has been depleted.