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Linux is a kernel, not an operating system. The Linux kernel is the part of the operating system that mediates access to system resources. It's responsible for enabling multiple applications to effectively share the hardware by controlling access to CPU, memory, disk I/O, and networking. Typically, "Linux-based" operating systems fall under GNU/Linux meaning they use the Linux kernel as well as the GNU system - a Unix-like operating system.
There are many widely-used Linux-based operating systems, called distributions or distros for short, in the world today. So many so, that it may be difficult to pick the right one for you. The biggest distros currently are Ubuntu, Fedora, CentOS, and Arch.
Ubuntu is used both as a user workstation and a server (they have separate iso's for each). It is by far the most dominate Linux-based distro and can be found in many offices around the world. It has the most support (https://ubuntuforums.org/) of any distro (rivaling Arch) and is an excellent choice for beginners looking to jump into the Linux world. It is fairly stable, but the updates come very slow.
Fedora is also a very popular workstation distribution. It provides very similar features to Ubuntu and comes with better support for virtualization and more frequent updates. Like Ubuntu, it is a good choice for those looking to run a Linux-based operating system for the first time.
CentOS, while popular, is typically meant for servers. Security on CentOS can be fine-tuned making it an excellent choice for running it on a server. I would not recommend using CentOS as a daily driver.
Arch Linux is mildly popular but notoriously difficult to manage. It is not recommended for new users looking to get into Linux, however the documentation (https://wiki.archlinux.org/) is extremely robust. There is a fairly large community of Arch users which make it easy to get help, but the setup time and ease is exceptionally difficult compared to Ubuntu.