All computers have the following essential hardware components

Input
The devices used to give the computer data or commands are called Input devices. Includes keyboard, mouse, scanner, etc
Processor
A processor is the logic circuitry that responds to and processes the basic instructions that drive a computer.
The term processor has generally replaced the term central processing unit (CPU). The processor in a personal computer or embedded in small devices is often called a microprocessor.
Short for microprocessor, the central processing unit in a computer. The processor is the logic of a computer and functions comparably to a human central nervous system, directing signals from one component to another and enabling everything to happen
Memory
Memory is the electronic holding place for instructions and data that your computer's microprocessor can reach quickly. When your computer is in normal operation, its memory usually contains the main parts of the operating system and some or all of the application programs and related data that are being used. Memory is often used as a shorter synonym for random access memory (RAM). This kind of memory is located on one or more microchips that are physically close to the microprocessor in your computer. Most desktop and notebook computers sold today include at least 16 megabytes of RAM, and are upgradeable to include more. The more RAM you have, the less frequently the computer has to access instructions and data from the more slowly accessed hard disk form of storage.
Memory is also called primary or main memory.
Storage
Computer storage is the holding of data in an electromagnetic form for access by a computer processor. It is also called secondary storage. In secondary storage data resides on hard disks, tapes, and other external devices.
Primary storage is much faster to access than secondary storage because of the proximity of the storage to the processor or because of the nature of the storage devices. On the other hand, secondary storage can hold much more data than primary storage.
Output
The devices to which the computer writes data are called Output devices. Often converts the data into a human readable form. Monitor and printer are output devices.