What is Linux?
Linux is an Operating System just like Windows or Mac OS. But unlike the other two, it is not owned by a single company and is open source. It means anyone can use it, share it, modify it, or do whatever they want to do with it without worrying about paying anything or piracy laws. You are the owner. Do whatever you want.
Note: The name of kernel (a small but important part of an OS) of Linux OS is also Linux.
How to download?
Unlike Windows or Mac OS, Linux is not owned by one company or person. So, there is no main website for downloading Linux. Many companies and persons provide their version/spin of Linux called a distro. Each distro has its own different name and slightly different features.You can download a distro from the website of creators of that distro.
Download links of some distros are given below.