A computer is a physical system that stores data, executes instructions and communicates with external devices.

Processor

The processor or CPU executes machine instructions. It performs arithmetic and logical operations, moves data and controls the order in which instructions run.

Memory and storage

Main memory provides fast working storage for programs and data currently in use. Persistent storage keeps data when power is removed.

These roles are different even though both ultimately store bits.

Input and output devices

Keyboards, displays, network interfaces, sensors, disks and many other devices let the computer exchange information with its environment.

A device does not normally execute application code directly. Hardware controllers and operating-system software coordinate communication between devices and programs.

Connections

Processors, memory and devices exchange data through electrical interconnects and protocols. The exact hardware may vary widely, but the functional structure remains recognizable:

input/devices ↔ processor ↔ memory
                    ↕
                 storage

Later system software builds abstractions over these physical components so that programs do not need to control every hardware detail themselves.