Linux is a family of free and open-source software operating systems built around the Linux kernel. Typically, Linux is packaged in a form known as a Linux distribution for both desktop and server use. (Wikipedia)
In simple words, its operating system like windows but with extra stuff to make android roms, servers, etc.
Ubuntu is an open source operating system for computers. It is a Linux distribution based on the Debian architecture.
Ubuntu ISO link: https://www.ubuntu.com/download/desktop
Rufus for making bootable USB: https://rufus.akeo.ie/