Fortran is a language which can be compiled using compilers supplied by many vendors. Different compilers are available for different hardware platforms and operating systems. Some compilers are free software, some can be used free of charge and some require the purchase of a licence.
The most common free Fortran compiler is GNU Fortran or gfortran. The source code is available from GNU as a part of GCC, the GNU compiler collection. Binaries for many operating systems are available at https://gcc.gnu.org/wiki/GFortranBinaries. Linux distributions often contain gfortran in their package manager.
Further compilers are available for example:
All these compilers support the Fortran 95 standard. An overview on the Fortran 2003 status and the Fortran 2008 status by various compilers is offered by the ACM Fortran Forum and available in the Fortran Wiki.