TEX consists – apart from the macrofiles – of two programs: initex and virtex.
With initex a format file (ending in `fmt') is created. To this end, this program initializes everything from the ground up, reads macros and separation tables and writes its memory contents in a relatively compact format, the format file. This procedure is usually called the ``dumping'' of a format.
virtex, a ``virgin'' version of initex, can now load this format file relatively fast, and can begin with formatting a text. Because of the now superfluous initializing virtex needs less memory than initex.
Now, what is this format file for?
TEX (more specificly plainTEX) and LaTEX themselves are no private programs, but only macro packages dumped in a format file, which you can load with virtex. In a shell you can define the following aliases to this end, e.g.