FORTRAN (the correct spelling), aka FORMULA TRANSLATION, is a language that has been around for many years. It originally was card punched and had peculiar characteristics due to that (with regard to continuing lines, etc.). These days, Fortran 90 (*) and newer editions work on all sorts of computers.
FORTRAN tends to be used for number crunching, modeling, etc. and has many libraries to provide support for such things.
see http://www.faqs.org/faqs/fortran-faq/ for more info.
(*) (Slightly pedantic note by AM) Up to and including the FORTRAN 77 standard the name was written in capitals. Since the Fortran 90 standard the name is written with a capitalised initial.
How does a FORTRAN application gain a GUI? The best-known commercial solution is Winteracter [L1 ]. Many of us, though, find managing Fortran programs with Tk more satisfying.
AM See also: Combining Fortran and Tcl in one program for another solution
Gustav Ivanovic See also calling Fortran routines in a DLL