Strictly speaking, a college offers a series of courses in one area. A university might be made of of a number of colleges.
"I attended the college of mathematics at the University of Wisconsin"
In normal speech, college may also refer to attending any university.
"After high school I am going to college."
Also in the U.S., "college" and "university" are almost interchangeable and there is not too much notice given to the difference. In fact, some schools that are universities have "college" in their name - Dartmouth College for one.
In other parts of the English-speaking world - England, for example - there is a distinct difference. A college is more of a specialized or trade school. A university would have more prestige, in general.