Look at Microsoft Windows as a translator that speaks and writes in a language easy for you to understand, that takes what you write or click on into the binary language , MS/DOS for most home computers, that the computer "thinks" in.
Before Windows, many different computer languages were used, most for specific applications, or needs. Basic, Fortran, there were many...and even for simple commands, things had to be "written" to make the computer understand what you were asking for. Windows automatically takes the simple things you ask or click on, and translates it into the complicated lines of commands that the computer needs to understand what you want. Many times a simple click of your mouse translates to many, many lines of programming.
Bill Gates and Steve Jobs were the first to ever come up with a universally easy to understand, and easy to use system for using a computer, and so as the first, and arguably the best, certainly the most versitile system, Windows has held it's share of the world's consumer market. Linux is free, "Shareware", but a certain level of programming is needed to tailor the system to what you wish, just like Mozilla. Geeks love them both, and for good reason, but they are a little above most people's programming abilities.
Mac has OS, and has made great strides towards being able to interface with Windows without a problem, and is getting better all the time. Windows just has had a bigger head start.