What is the difference between real number and complex number? When complex numbers are plotted on graph why the Y axis is called imaginary part?

complex numbers have both a real and imaginary part, the imaginary part is a multiple of \sqrt{-1}, more commonly represented by i.

Complex numbers can be represented on an Argand diagram in the way that you mention, with the x axis showing the magnitude of the real part, and the y axis showing the magnitude of the imaginary point.

For example, the complex number P = 2 + 3i would be shown on the Argand diagram as a point at (2,3).