Like Windows, Linux – a UNIX derivative – is an operating system, i.e. the software underneath all the other software on a computer.
The advantage of Linux is that it is an open source software, with many programmers contributing to the source code.
Although Linux might be similar to other operating systems, i.e. it offers the same type of applications you would use on Windows, for example, it is different in the sense that it consists of various distributions, which include different software options.
Many, if not majority of the web pages you browse online are generated via Linux web servers, offering a stable and secure environment.