Classes are hard. When people hear you go to Princeton, most assume you're some sort of genius. That may be, but most likely you don't feel like it most of the time when you're at Princeton, because everyone is so academically driven that even Pass/Fail classes aren't a joke. When I came back home after my first semester at Princeton, lots of my friends at other schools were talking about how much easier college was compared with high school. Unless you went to a ridiculously tough high school, this will probably not be the case at Princeton.
Given the academic rigor, Princeton students do study a lot, partly because of the amount of work their given, and partly because most of them just seem to be ambitious, driven individuals. There are tons of libraries on campus (and the library system is immense/impressive...students here definitely take it for granted) and students can also be found studying in dorm common areas, cafes, and the campus center. On average most students work 3-5 hours per night, but sometimes less, sometimes more. A lot depends on your major. Humanities students tend to have lots of reading, often seemingly impossible to complete, whereas science major usually have lab reports and problem sets. Finals take place after winter break, with a week's reading period and then two weeks for exams. This can be a stressful period, but it's great having to class to focus on finals work. It is easy to forget a lot over winter break though.
The student/professor relationship is generally very positive, and becomes more personal when you begin to focus in on your preferred department for a major or certificate (the Princeton equivalent of a minor). While there are many large intro classes, there are also plenty of smaller seminars, and these are usually the majority once you get past the intro levels. The large classes usually break into what's called precept once a week, a one hour small discussion group to cover the material in a smaller setting, led by a TA or the professor. This is good to keep you from getting lost in large classes, but it also means you can't totally zone out in these, because you'll be expected to contribute to precept discussion, which makes up a part of your class grade.
Popular majors are always Politics, Woodrow Wilson School, Economics, and History, and these are all stellar departments. All of the smaller departments have awesome faculty as well though, and students sometimes benefit from the extra attention they can get from majoring in a less common field, especially when it comes time for the two JPs (junior papers) and the daunting senior thesis they must complete before graduation.
You don't have to declare a major until sophomore spring, which is great for those like me who enter Princeton clueless as to their academic goals. I've settled on Comparative Literature, because it has the potential to incorporate many of my interests, such as foreign language, translation, creative writing, literature, film, and art. One way to figure out what you're interested in studying is simply by completing the required courses. All freshman have to take a writing seminar, and although most people I know seem to have hated this, my writing seminar, on the culture of consumption, has been one of my favorite classes so far. There's also a foreign language requirement and a number of "distribution requirements," which compel you to take classes in a number of broad categories. The hardest of these to complete for a hardcore humanities person like me are the one Quantitative Reasoning (mostly math classes) and two Science and Technology (with lab!) classes. These are good opportunities to use the four PDF classes you're allotted. I took an Astronomy class like this last spring as my math, and science I got out of the way with Intro to Psych and a class on Lasers I'm currently taking. All in all, the requirements aren't too painful and can introduce you to a new field of interest, or at least provide you with a funny story about bumbling cluelessly through lab.
There are many career-driven students on campus (many seem to be aspiring analysts and i-bankers), but probably an equal number like me who still haven't quite figured out what the future holds for them. Even the career-driven seem to branch out and take the opportunity to explore other interests while at Princeton though, so people in your classes usually range from department majors to the random finance student who has always wanted to take a ceramics class.
Princeton is quite a competitive place, and getting in is far from the last time you feel the strain of competition. While classes aren't aggressively competitive, the grade deflation policies still give everyone the feeling that its necessary to outperform their peers. Sophomore year, many students compete to get into the Woodrow Wilson foreign policy school. Others compete for summer internships or grants. Even fun at Princeton seems competitive at times. Many extracurricular groups require audition and acceptance--from a capella, dance groups, the business society, and theater, to just being a campus tour guide. In addition, half of the eating clubs that many students join during sophomore year require "bicker" (similar to rush) to gain admittance.