The three basic subsystems of a computer system are the central processing unit (CPU), the random access memory (RAM) and the storage. Each subsystem can be broken down into smaller systems, but these are the three major subsystems of any computer.
The CPU is the component that processes instructions. It receives input from the computer's user, operates applications and the operating system and processes data to provide output.
RAM provides the operating memory for the computer. The more processes a computer needs to run at a single time, the more RAM it needs. Operating processes from RAM is much faster than running them from a disk or hard drive.
A computer's storage permits it to permanently store files and data. The hard drive is a common storage device that varies in capacity.