Most TeX documents at a particular site are designed to use the standard paper size (letter size in the United States, A4 in Europe). The Dvips program can be customized either sitewide or for a particular printer.
But many documents are designed for other paper sizes. For instance, you may want to design a document that has the long edge of the paper horizontal. This can be useful when typesetting booklets, brochures, complex tables, or many other documents. This type of paper orientation is called landscape orientation (the default orientation is portrait). Alternatively, a document might be designed for ledger or A3 paper.
Since the intended paper size is a document design decision, not a printing decision, such information should be given in the TeX file and not on the Dvips command line. For this reason, Dvips supports a `papersize' special. It is hoped that this special will become standard over time for TeX previewers and other printer drivers.