GNU ("GNU is Not Unix", a recursive acronym) is a large project started by Richard Stallman, the inventor of free (as in freedom) software, running since 1983 with the goal of creating a completely free (as in freedom) operating system, along with other free software that computer users might need. The project doesn't tolerate any proprietary software. The project achieved its goal of creating a complete operating system when a kernel named Linux became part of it in the 90s as the last piece of the puzzle -- the system is now known as GNU/Linux. However, the GNU project didn't end and continues to further develop the operating system as well as a myriad of other software projects it hosts. GNU gave rise to the Free Software Foundation and is one of the most important software projects in history of computing.
The mascot of GNU is literally gnu (wildebeest), it is available under a copyleft license.
The GNU/Linux operating system has several variants in a form of a few GNU approved "Linux" ditributions such as Guix, Trisquel or Parabola. Most other "Linux" distros don't meet the strict standards of GNU such as not including any proprietary software. In fact the approved distros can't even use the standard version of Linux because that contains proprietary blobs, a modified variant called Linux-libre has to be used.
GNU greatly prefers GPL licenses, i.e. it strives for copyleft, even though it accepts even projects under permissive licenses. GNU also helps with enforcing these licenses legally and advises developers to transfer their copyright to GNU so that they can "defend" the software for them.
Although GNU is great and has been one of the best things to happen in software ever, it has its flaws. For example their programs are known to be kind of a bloat, at least from the strictly suckless perspective. It also doesn't mind proprietary non-functional data (e.g. assets in video games) and their obsession with copyleft also isn't completely aligned with LRS.
GNU has developed an almost unbelievable amount of software, it has software for all basic and some advanced needs. As of writing this there are 373 software packages in the official GNU repository (at https://directory.fsf.org/wiki/Main_Page). Below are just a few notable projects under the GNU umbrella.
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