See Universities are autonomous body. It has higher education courses like Masters/PhD etc as well as it can affiliate colleges under there prescribed syllabus or can be non-affiliating. Now to explain it say you want to do a course in Presidency University, Kolkata. Now this is a university that have both Bachelors Masters as well as phD program but it doesn't affiliate colleges. But if you want to do a Bachelor course under Calcutta University then you will have to get admitted to a college affiliated by CU.

Now depending on the institute you want to take part in will mean which you will get admitted to. For example, if you want to study BCA in Techno India University then you will be admitted to a university but if you want to do BCA under a affiliating university then you have to get admitted to a college.

I hope this answer helps. All the very best for your future endeavors!