The Personal Computer
The picture shows the basic architecture of a personal computer. The
'intelligence' is provided by the Central Processing Unit (CPU) but the
CPU gets its instructions from the memory and from external inputs via the
input/output (I/O) circuits.
A computer program consists of both data and instructions on what to do with that data. As all of the computer's circuits only understand numbers, both the data and the instructions have to be in numerical form.
The CPU will work progressively through memory interpreting memory locations as either data or instructions and, as one number may look very much like another, it is the CPU's responsibility to keep track of whether to treat each number as a number (ie data) or as an instruction.