OpenTemporary[ ] opens a temporary file to which output can be written, and returns an OutputStream object.
OpenTemporary is often used in conjunction with Put and Get as a way of preparing data that is exchanged between Mathematica and external programs.
OpenTemporary always creates a new file, that does not already exist.
On Unix systems, OpenTemporary typically creates a file in the /tmp directory.
The global variable $TemporaryPrefix gives the base of the file name used by OpenTemporary.
See The Mathematica Book: Section 2.11.2.
See also: Close, Run.