Temporary files may be created by computer programs for a variety of purposes; principally when a program cannot allocate enough memory for its tasks, when the program is working on data bigger than architecture's address space, or as a primitive form of inter-process communication.
Modern operating systems employ virtual memory, however programs that use large amounts of data (e.g. video editing) may need to create temporary files.
Most operating systems offer primitives such as pipes, sockets or shared memory to pass data among programs, but often the simplest way (especially for programs that follow the Unix philosophy) is to write data into a temporary file and inform the receiving program of the location of the temporary file.
Some programs create temporary files and then leave them behind - they do not delete them. This can happen because the program crashed or the developer of the program simply forgot to add the code needed to delete the temporary files after the program is done with them. The temporary files left behind by the programs accumulate over time and can take up a lot of disk space. System utilities, called temporary file cleaners or disk cleaners, can be used to address this issue.
Usual filename extensions for temporary files are .TMP
On Unix, temporary files can be safely created with the mkstemp library function, or with the mktemp program.