In the US a college and university are essentially the same thing. They are both institutions which give degrees. In commonwealth nations the terms are more distinct. A college can be a school affiliated with a university – the college prepares the student for the degree and the university with which it is affiliated gives the degree.
Another way to describe the difference in the US is a college offers
a collection of degrees in one specific area while a university is a
collection of colleges. When you go to a university you are going to
be graduating from one of their colleges, such as the business
college. A Community College is different from both in that it
cannot grant a bachelor's degree.
Some “colleges” in the UK are really secondary schools. One famous
example is Eton College, where students typically enter at age 13.
In Australia and New Zealand, “college” means high school.