Millions of years ago, before the dawn of civilization, cavemen went into the office each morning and used a program known as DOS. DOS stands for Disk Operating System, and it was the operating system of choice for home and office computers during the 80s and the first half of the 90s. Visually, DOS has almost nothing in common with Windows (that the average user would notice; many things were similar under the hood), but it is the first step in understanding what Windows is, because it shows what an operating system was like before Windows came along, and it's also because MS-DOS, Microsoft's version of the DOS operating system, is where Microsoft got its start.
When you started a DOS-based computer, you were greeted by a simple command prompt, the famous C:\>. In order to use the computer, you had to type in commands. A mouse was not generally part of the experience of using a DOS-based computer, at least not in the operating system - the keyboard was your primary guide. To accomplish anything, you had to type in commands. For example, to change directories, you had to type in the command cd and then the name of the directory.
As you might guess, this was far more difficult to use than Windows. To accomplish a task, you had to know exactly the command you needed and exactly the location of what you wanted to run, open, copy or change. If any part of what you typed was wrong, you'd receive an error message.