Showing complex numbers as a point on an Argand diagram allows further properties to be found.
The modulus of a complex number is the length of the line which joins it to the origin (0, 0). From the diagram below, using Pythagoras' Theorem, it can be seen that:
The modulus of the number a + bi r = √(a^{2} + b^{2}) 
The modulus of a complex number z is written IzI
The argument (θ) of a complex number is the angle formed at the origin between the positive xaxis and the line which indicates the modulus. This angle is usually measured in radians, and is given for values between π < θ ≤ π.The argument of a complex number z is written arg(z)
From the diagram it can be seen that:
a = r.cos θ and b = r.sin θ
Thus the complex number a + bi can be written:

cis is an abbreviation for cos θ + i sin θ.
Summary
a + bi is called the RECTANGULAR or REALIMAGINARY form r cisθ is called the POLAR or MODULUSARGUMENT form. 
e.g. For the complex number z = 1 + i Modulus IzI = √ (1^{2} + 1^{2}) = √2 Arg(z) = cos^{1} (1/√2) = Therefore 1 + i = √2 cis 