Recall the following definitions:
A relation is a correspondence between 2 sets.
Algebraically, you can think of a relation as a set of ordered pairs (x,y). Once special type of relation is called a FUNCTION.Every function is a relation,but not every relation is a function.
A function is a relation for which each and every input gets mapped to ONE and ONLY ONE output.
The following applets help further illustrate this concept for functions represented graphically.

If a relation is a function, each input has ONLY 1 OUTPUT!

This relation is NOT A FUNCTION. Slide to see why.