It is like a cross between windows and a Mac
In strict words, linux is 'just' a Kernel for an operating system. But commonly speaking, "linux" refers to global operating systems, based on this kernel, with several versions (redhat,slackware,debian,gentoo,ubuntu amongst lots of others).
Linux distributions are fully functionnal and independant operating systems, much more than just "a cross between windows and MacOS". One of the major property of linux operating system is its user licence : it's based on open and free software, which means everybody can enhance or adapt it, and that you may install it on your computer legally without paying anything.
Beside of that, linux os have lots of technical properties, but that would be much too long to describe, and would not be useful and adapted here
but you can find tonns of information with google about it !