Original: The FORTRAN defined by the first standard became known as FORTRAN 66 (although many continued to refer to it as FORTRAN IV, the language upon which the standard was largely based). FORTRAN 66 effectively became the first "industry-standard" version of FORTRAN.
Simplified: Several revisions of the language have appeared, including the very well known FORTRAN IV (the same as FORTRAN 66), FORTRAN 77 and Fortran 90.