Part 2 of 2. d) Schools that only teach undergraduates, only grant bachelors degree, and have no other schools, are called "colleges." Some well-known schools of this kind include Amherst College, Haverford College, Vassar Collage. e) Because people think of universities as bigger and "better" than colleges, in recent years there has been a tendency for small colleges to call themselves "universities" if they can find any excuse to do so. f) Just to make things totally confusing, there are two big, famous, excellent full-sized research universities that, for reasons of history, prefer to call themselves "colleges:" Dartmouth College and the College of William and Mary. g) The terminology vaguely echoes those of medieval universities in Europe, which were collections of colleges. Oxford and Cambridge have numerous _undergraduate_ colleges (Balliol College, Kebel College, etc.) In the U.S. most universities contain a _single_ large college, but there are exceptions--I think Yale has a collection of separate undergraduate colleges.

Part 1 of 2. In the United States: colleges and universities are both institutions of higher education, following "high school" (secondary school). Students enter at about age 18. The terminology is inconsistent--there are no rules--but it is generally true that a) a "college" is a school where students enter at about age 18, study for four years, and graduate with a "bachelor's degree." The name of the degree would be either Bachelor of Arts or Bachelor of Science, and students often have a "concentration" or "major" in a narrower field. I am a Bachelor of Science in Mathematics, for example. That makes me a "college graduate" with a "bachelor's degree." During the time they are studying, the student at a college are not yet graduates and are referred to as "undergraduates," and the college is "an undergraduate school." b) Traditionally, a "university" has more than one "school." One of them is a "college" for undergraduates. The others are "graduate schools" that grant advanced degrees, such as the master's and doctorate (Ph. D.). They often have schools in specific professional fields--a law school that grants a doctor of laws or J.D. degree, a medical school that grants an M.D., etc. c) Thus, my father, for example was graduate of "Columbia College," which is the undergraduate school that is part of "Columbia University."

For me (an American) they are exactly the same.

The Harry Potter analogy is actually a very good one. The only difference I can think of is that 'college' is only refers to accomodation during tertiary education. I very recently graduated from a four-year bachelor degree at ANU. I stayed at a college (called 'Burton and Garren Hall') for the first year. There they organised several social events, and there were sports competitions between the colleges. For the next three years, I lived elsewhere in Canberra, and wasn't a member of a college. Lots of other students studying there never joined a college at all.

" So when someone says I went to King's College at Oxford the only difference between them and a person who went to a different college is where they stay ? All of their courses and everything else are (or could be) the same?" - Yep, that's exactly right.