In others, a college is a part of a university. The educational system of the United States is very large and diverse. It has the advantage of being very flexible. It sometimes has the disadvantage of being confusing, even to students from the United States. There are, of course, many differences.
Graduate programs are highly specialized and much more advanced than undergraduate programs.
Undergraduate classes are usually much larger and less individualized. While some undergraduate programs require a senior project or a similar activity, graduate studies are much more research oriented. It is comparatively easy to change undergraduate major subjects and to move to a different school during a program. It is much more difficult to do so in graduate programs, as they narrowly specialized. Graduate programs in, say, Psychology may be very different from one university to another.