The two most common types of windows are either the clear panes of glass you can look through in a building or vehicle, or the series of popular operating systems published by Microsoft.
Expanding on the latter, Windows was developed as a graphical user interface for their previously popular MS-DOS operating system. DOS required users to have a good deal of technical savvy and memorization of text-based commands, whose parameters could become very complicated to do relatively simple things, like moving files or sorting them by different criteria.
The Windows solution for that was to simply give the user menus they could easily navigate and basic commands to click on that generated the same complicated input in the background, but with an interface easy enough that anyone could work with it.
Windows 3.1 was the first notable step in the series, and Windows 95 expanded significantly on that formula, opening the doors to faster 32-bit computing that would be developed further in later iterations of the OS, including Windows 98, ME, XP, Mobile, Vista, and network-focused versions like Windows Server and Windows NT.
Windows' main competition comes from Apple's Mac OS and the various open-source Linux operating systems. Despite user complaints about bugs and shoddy performance in virtually every version of Windows, it remains the most popular OS in the world.