This is a nice question, and it has a broad answer. Instead of speaking about a computer, I prefer to speak about a computer system, because it really works as a complex system, where all and every component has a specific and important task.
In general we can say that computers have a CPU (Central Processing Unit), a controller, different types of memories (ROM or Read Only Memory, a RAM or Random Access Memory; a BIOS which is a type of ROM that is used by the computer to establish basic communication when the computer is first turned on; a caching or extremely fast RAM that connects directly to the CPU and allows the storing of frequently used data, a virtual memory, etc).
The computer also has different kind of BUSES to establish communication with the several internal and external devices; the external devices include the monitor, the keyboard, the mouse, the printer, speakers, microphone, and any devices that can help in your kind of work. The computer system also has different ports or connections to communicate with the external devices, and removable storage (devices that allow you to add new information to your computer very easily, as well as save information that you want to carry to a different location).
The CPU is like the brain of the computer, everything that a computer does is overseen by the CPU. The controller and the CPU work very close, as the brain and the nervous system do in the human body. The computer receives information through the connections and the external devices. It is the controller that is in charge of controlling these activities coordinating the external devices, the ports, and the internal buses which are going to access the CPU and the memories.
All inside the computer works with electrical signals. Even the information that we send in and out of the computer is made of bits, numbers that represent different levels of voltage. When we type a number or a letter in a keyboard, the computer system translates this character into a string of bits, and this is the language that the computer understands. That is why a computer can not do anything out of the human control, but she can do it faster that the human brain and this is the reason we like and need computers very much.
In order for the computers to work with the rest of the world, there are rules, protocols, codes, and all kind of agreements that establish the way in which computers must communicate and understand the information. All these rules have been evolving through the years according to the needs of the people and the different devices that make a computer system. Then you hear about OSI layer 7 communication protocols such as: FTP (File Transfer Protocol), TELNET, SNMP (Simple Network Management Protocol, and SMTP (Simple Mail Transfer Protocol).
The computer understands instructions through programs written in languages that have been developed especially for them. The programs follow syntax rules and are designed to control the computer as it performs specific functions. Some of the functions are required so that the computer can provide a working environment for the user and other functions are utilized by word processors, spreadsheets, email, databases and other applications when they are needed.
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