To me, a university is bigger than a college. More students, more departments, more of a grad school presence, etc. While there is no rule about such things, that's what comes to my mind.
In the U.S., those terms are almost interchangeable, with universities that don't award the doctorate and colleges that do. In many other parts of the world, the term "college" connotes a non-degree-granting, usually tertiary, school.