When we find the limit of a function f(x) as x goes to infinity, we're answering the question "What value is f(x) approaching as x gets bigger and bigger and bigger...?''
A rockin' example of this is
As x gets bigger and bigger, 
gets closer and closer to zero. We can get a feel for this by making a table:
Since 
is approaching 0 as x gets larger,
Similarly,
This makes sense on the graph, since the bigger x gets, or the more negative x gets, the closer y gets to zero:
Graph of 
:
If we had some other constant in the numerator besides 1, the limit would still be 0. We would still be dividing a constant by larger and larger numbers. Therefore,
In fact, whenever we have a rational function f(x) where the degree of the numerator is less than the degree of the denominator,
Sometimes the limit of a function as x goes to ∞ is undefined. Take f(x) = sin x. As x goes to infinity, sin x just keeps bouncing between 0 and 1 without really ever honing in one number. That limit doesn't exist.