Historically there have been 2 types of Computers:

1. Fixed Program Computers - Their function is very specific and they couldn't be programmed, e.g. Calculators.

2. Stored Program Concept - These can be programmed to carry out many different tasks, the application is stored on them, hence the name. 

Modern computers are based on a stored-program concept introduced by John Von Neumann. In this stored-program concept, programs and data are stored in a separate storage unit called memories and are treated the same. This novel idea meant that a computer built with this architecture would be much easier to reprogram.

The basic structure is like this:


It is also known as IAS (Institute for Advanced Study) computer and is having three basic units:

1. Central Processing Unit ( CPU ) 

2. Main Memory Unit ( MU )

3. Input / Output Device