These are the answers that were given above. Just as we can plot real numbers as points on a line, we can think of complex numbers as lying on a plane. What is most useful about this visualisation of the complex numbers is how addition and multiplication behave.
Multiplication is a bit more difficult to see.
Before we go on, it is useful to introduce another means of specifying points in the plane. Suppose that X is any point in a plane and suppose that O is the origin. This angle and distance are known as the polar coordinates of X. Multiplication corresponds to adding the angles of the two points and multiplying their lengths. Understanding multiplication helps us understand other operations such as taking the square root. For the sake of simplicity, first consider the case in which the number we are interested in has a distance of 1 from the origin.