Concept of Relative Motion

What is relative velocity and how can we understand it in a easy manner.

Relative Motion of two bodies :: Suppose if two bodies are in motion and are separated by a  distance then the concept of relative motion is useful.

Relative Velocity:: Suppose A and B are two objects which are moving with velocities 

X m/sec and Y m/sec then we can apply relative velocity of the two bodies . we can analyse the motion of the bodies.

Even we have to look for the motion of the objects A and B whether they are travelling in a straight path (1-d)or not(2-d / 3-d).

In 1 - D plane the explanation can be understood like this...

Suppose if A and B are approaching each other then the distance between them decreases as because their relative velocity is more compared to their original velocities.

Suppose if A and B are travelling in the same direction then the distance between them increases or decreases depending upon the velocities X and Y.

The whole situation can be viewed as