Before those really smart math guys start throwing equations at you, think of a person who is told to stand in front of a wall and told to move half the distance to the wall every hour. Intuitively, you can see that he will never reach the wall, although he is constantly approaching it. A graph of this would show a curve approaching an axis but never touching it.
(My apologies to the real math guys out there--I was just a pre-med student).