Academically, you will see the following differences:
- You will be much more responsible for your own academic performance in college. Your teachers will expect you to be self-motivated, and will do far less things to make sure that you do not fall behind, like in high school (e.g. tracking attendance, more frequent smaller assignments, etc.).
- At most colleges, there will be far fewer common courses that most students take. Your courses will be based heavily on your major and interests, which may be disjoint from those of many other students, whom you may never see in the same courses.
Socially, it depends a lot on your high school and college. However, the major difference, at least in the US, is that many college students who go to college right after high school will be transitioning to legal adulthood in most respects (except for purchase and possession of alcoholic beverages, which must wait until senior year for most traditional college students, although restrictions are widely ignored) at around the time they enter college. Some will be living away from their parents (or boarding school supervision) for the first time, so they must manage the rights and responsibilities that are new to them on their own.