The GNU Project was launched in 1984 to develop a complete Unix-like operating system which is free software: the GNU system.
GNU's kernel wasn't finished, so GNU is used with the kernel Linux. The combination of GNU and Linux is the GNU/Linux operating system, now used by millions. (Sometimes this combination is incorrectly called Linux.)
There are many variants or “distributions” of GNU/Linux. We recommend the GNU/Linux distributions that are 100% free software; in other words, entirely freedom-respecting.
The name “GNU” is a recursive acronym for “GNU's Not Unix”; it is pronounced g-noo, as one syllable with no vowel sound between the g and the n.