In general, a college is a smaller institution and it usually only offers bachelor's degree-level programs. A university is usually a larger institution and they usually also offer Master's and phD programs. Many also offer law or medical degrees. Your confusion might stem from the different ways the words are used in British English and American English. In the US even if you go to a big university, you're still in college. In the UK even if you go to a small college, you're still at university.