The GNU Operating System was started in 1985 by Richard M. Stallman as an effort to create a UNIX Compatible OS composed of entirely Free Software. A working system was available around 1991, with the only missing piece being the kernel. Linux was begun around that time and provided the much needed kernel for the GNU Operating System, creating GNU/Linux (The GNU Operating System with the Linux kernel). The GNU project also has its own kernel-like project named The Hurd.
The GNU Project provides the standard C library, compilers, and core POSIX utilities needed to have a UNIX compatible OS.