It depends on the college/university and what you’re studying. (In the US they’re used interchangeably, but in other countries those words mean very different things.) I went to a small private college in the same city as a huge public university with a reputation for being a “party school” (which was also where most people from my college went Thursday nights; since Friday classes are rare in college and a lot of students work on weekends, Thursday night is party night). Organization varies from one college to another, but at the one I went to there were different “schools” for each area of study (math, sciences, arts and humanities, and business were the big ones). Each school has a dean in charge of it, but they’re all organized under the same college, with the president and the board of trustees in charge of the whole thing.
I never went to the big college parties, since I’m more of a goth nightclub and small punk venue kinda guy, but you always heard the wildest things about them. College parties have a big reputation for not being safe to go to alone, and I have plenty of friends who can confirm that. There are also a lot of different clubs on campus, even in smaller universities, so no matter what your interests or political leanings (unless they’re extreme), you’ll probably find something that fits your interest. They’re a lot more relaxed than high school extracurriculars even though you’re usually planning more activities that involve the entire school community. I remember in high school, Student Association was this super serious thing with strict rules of conduct. In college, they had higher standards (you had to do more than show up at meetings and sign paperwork) but meetings were a lot more interesting. Also in college, we had Student Association merchandise that said “Stud Ass” and I think that’s hilarious.
The more academic side varies depending on what you’re studying, but you’ll also be taking some classes outside of your major early on unless you’re going to a very specialized college (such as art school). Each major has its own challenges (art majors have a lot of big projects, the hard sciences involve a whole lot of math and time in the lab, social sciences and humanities are super writing-intensive, etc.), so no matter what you’re in for a lot of hard work. Class sizes depend on the school. Where I went it varied from 8-25 people in a class, but that’s something you’ll only see in small colleges. I’m currently a teaching assistant at a larger university and for an introductory class, you’ll get around 300 students crammed into a single lecture hall. The larger the class size, the less discussion and the less individual attention, but it also means that nobody cares if you go to class as long as you show up for exams. That’s the thing about college in general: you’re responsible for your own work. If you fail a class in high school, people blame the teacher (though maybe not so much with homeschooling). If you fail in college, that’s on you. You can usually retake a failed class once or twice, but no more than that.
Your schedule is going to vary. You’ll usually have classes that either last an hour and a half and happen twice a week or last three hours and happen once a week. If your class involves a lab portion, it will either last longer or happen more often, depending on the professor’s preference. You can choose most of your classes as long as they fit educational requirements. Colleges have more than one academic building too, so you’ll probably be walking around on campus a lot if you have multiple classes in a day. Usually you’ll take 4-6 classes in a single semester as an undergrad and 2-3 per semester in grad school.
Your friends may be from all different programs. I studied forensic psychology in undergrad, but most of my friends were art and philosophy students. I’m not sure how that happened, but it did. Graduate programs are a little different, though. The programs themselves are smaller, for one. While you might have a few hundred psychology undergrads, a lot of master’s programs will only allow up to 15 people, and PhD programs even less, because universities have to actually pay their graduate students and provide options for funding research. My program right now has a cap of 10, and they usually don’t even accept that many. My cohort for it has five people including me, and we’re all in the same classes. As a result, we’re a much closer-knit group than you’d find in an undergraduate program. I bring this up because a college will have a combination of graduate and undergraduate students, with people of all ages involved, and they don’t all stick with people in their own year.
Overall, the social atmosphere is pretty chill and age doesn’t matter so much in friendships. I mean, I’ve gone drinking with professors before, so… yeah, it’s a more relaxed atmosphere, but with a lot more work.