It must be told what to do in a language it understands. The computer needs a set of instructions called an operating system. Even with an operating system, a computer can't do much. It can't send e-mail, balance your checkbook or cruise the Internet. For that, it needs special programs designed to work with the operating system.
Several companies make operating systems.
In the 1980s, it was a fiercely competitive market, with IBM, Apple and Microsoft all trying to sell their systems. But in the early 1990s, through a combination of aggressive marketing by Microsoft and blunders by its competitors, Microsoft pulled ahead with its Windows operating system. Their choice of footwear will not affect their ability to play together. But companies need computers that can talk to one another. So once they started buying Windows, they kept buying it, and Microsoft's small, early lead grew into a huge one.