By the book CPU is a brain of a computer that is the common definition of CPU. Some people believe or there knowledge about CPU, it is the thin metal sheet box where found inside the hard disk, mother board and many components of your system. Actually the thin metal box like is only the case who is protecting the inside components of your system and to avoid confusion the CPU term tends to drop also in the market refers to the package that includes all the components inside the case.
The real CPU found inside a case that is attach to the mother board of your system. This chips processing billions of transactions per second. This processing of information or ratings that measures a certain number of information per second called the clock speed. If you have a processors of 3.0GHz means three billions of data is being processed per second.