Windows is a single product (albeit one with several editions) produced by a single firm, namely Microsoft.
Linux is a ‘family’ of operating systems which use the same kernel (the ‘brain’ of the system), but is available in hundreds of different forms, each known as a distribution. A distribution consists of the kernel, the operating system itself, some method of displaying information (usually a graphical user interface that is similar to the on-screen look of Windows or the Mac operating system); it will usually contain some programs as well.
(Note that the term ‘Linux’ is commonly used to refer to any and all operating systems based on the Linux kernel.)
Having so many different versions of Linux offers much more flexibility, but can be extremely confusing for new users.