A modern computer has the following hardware components: input/output devices, processing unit, memory unit and secondary storage. The memory unit is the principal storage of the computer. All the data and instructions that the computer needs at a moment are stored here.
How It Evolved
The earliest types of memory were electromechanical relays, mercury-filled tubes and transistors. Now the memory is in the form of a semiconductor or silicon chip.
A computer memory unit is the storage where data and instructions go when entered into the computer.
The data and instruction need to be stored so that they'll be available when required and can be used repeatedly without having to enter them each time through an input unit.
Memory units nowadays are measured in megabytes; the usual computer may have 64 MB, 128 MB, 512 MB or more. Some have memories in gigabytes. The higher capacity, the better computer performance.
There are two types of memory: The primary memory or random access memory (RAM), which is temporary storage. And the secondary memory or read-only memory (ROM), which is permanent storage.