The four basic functions of a computer system are input, processing, output and storage. These four functions are collectively known as the IPO+S model and are used to teach the fundamentals of information systems.
Input collectively refers to all the resources used in creating a program. This usually consists of a programmer entering code into a computer with a keyboard.
Processing collectively refers to all of the internal functions the computer uses to control and make sense of information. The most basic components are a control unit, an arithmetic logic unit and random access memory for temporary storage. This represents a very simple model of a computer. Modern computers will add other processing components such as a multitasking operating system.
Output is the display of information to the user. This can occur through the monitor, by way of audio through the speakers, or on paper via a printer. Output can also be written to a storage device like a CD-ROM or flash drive.
Storage consists of any means by which data can be permanently saved. Some examples include internal and external hard drives, floppy disks, CDs, DVDs and memory sticks. This component was not always present in the IPO model as it is not strictly necessary for a basic computer to function, but has been added as permanent physical memory has become a standard part of all modern computers.