A CPU (or "central processing unit" as it is technically referred to) is a small piece of electronic equipment that is designed to operate other pieces of electronic equipment. Essentially, it is a small circuit that carries out basic instructions that tell whatever it is inside of how to operate. For example, in a computer the CPU's job is to load the operating system, which is the piece of software that allows you to do things like access the Internet and write Word documents.
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A CPU will execute a total of four steps before finishing its job and operating whatever piece of electronic equipment you've attempted to turn on. The first of those steps is referred to as the "fetch" step, where the CPU will look into its memory for a list of instructions of what to do next. After that is the "decode" step, where the total instructions are all assigned to different, relevant parts of the CPU itself.
The next step is referred to as the "execute" step, and is where all of the instructions are executed by the CPU. To return to the example of a personal computer, it is at this point that the CPU has loaded your operating system. At this point the CPU's job is essentially over. The last step is referred to as the "writeback" step, and is where the CPU stores data of the results of the previous step to its memory should the need for a record of such data arise in the future.