GNU/Linux is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. The GNU part of GNU/Linux was created by Richard Stallman (shown in the video below). GNU/Linux is typically packaged in a GNU/Linux distribution.
Virtually all of the GNU/Linux distributions are Free Software, and this freedom has nothing to do with price. The freedoms provided with Free Software are the freedoms to (1.) run the software (2.) study the software (3.) change the software and (4.) re-distribute your changed version of the software.
There are many prominent GNU/Linux Ditributions. Here are a few of the very popular ones:
- Manjaro: https://manjaro.org/
- Mint: https://linuxmint.com/
- Pop!_OS: https://pop.system76.com/
- Ubuntu: https://ubuntu.com/
- Debian: https://www.debian.org/
- Fedora: https://getfedora.org/
- OpenSUSE: https://www.opensuse.org/
- CentOS: https://www.centos.org/
You can find a complete list of GNU/Linux distributions here: https://distrowatch.com/