For those that don't know, Linux is a kernel. A kernel is a backend of any OS. The kernel is what communicates with hardware. It also handles other tasks that are vital for the OS to function. Any software that runs on top of that kernel makes it an Operating System. Ubuntu is software that runs on top of the Linux kernel, making it a Linux, based OS.
One major and well known example of Linux is the distro Ubuntu. Other desktop distros are Linux Mint, Fedora, RedHat, etc. There are thousands of customized distros of Linux out there, but these are just some of the well known ones.
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If you want to know more about the Linux kernel, go here.