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Without an operating system, a computer is only a box of electronic parts. It takes an operating system to turn a computer into a functioning machine for work or play. The operating system is the software that starts the basic functions of a computer, displays texts on the computer’s monitor, accesses the Internet, and runs applications — it transforms the computer into a powerful tool. There are many kinds of operating systems, but only a few have really captured a wide audience. Some popular operating systems are used on computers that fit on your desktop. Others are used on more powerful computers called servers that are accessed by multiple users through a computer network. Some, like UNIX/Linux, are used on both desktop and server computers. And some are embedded in a variety of electronic devices from cell phones to photo kiosks.
The word “it” (l.6) refers back to “The operating system” (l.3-4.)