I'm a college junior totally stressing right now in the middle of finals. Most of my friends aren't stressed out because they're turning in papers they already wrote for other classes or sharing papers with each other so they each only have to do some of the work. Some people are just copying (or buying) papers from online. This happens all the time, and I don't know anyone who's gotten caught, so I'm really tempted to take the easy option this time. What counts as plagiarism?
Playing with Plagiarism
All of it counts as plagiarism, and none of it is worth it. Don't do it. I'm going to say a lot more on the subject, but in case you decide to spare yourself the lecture, I'll say my most important point again: Don't do it. Don't copy someone else's work and pass it off as your own. Don't cheat yourself out of the education you (or someone else close to you) is undoubtedly paying a lot of money for. Don't give into your friends' idiotic peer pressure to have fun during finals instead of doing the work you're in school to do.
I encourage you to look at your university's academic policies for their definitions of plagiarism. While you're there, go ahead and peruse the consequences of cheating, which may include anything from failing the paper to failing the class to being kicked out of school. Even taking an incomplete or turning in the worst paper you've ever written is preferable to passing off someone else's work as your own. Plagiarism is a stupid thing to do and is certainly against your school's policies.
Even if no one you know has gotten caught, give your professors and your university some credit for knowing that students are cheating. The problem of plagiarism is so rampant that I wouldn't be surprised if your professors spend more time googling the papers they receive than actually grading them. After all, why should they bother commenting on a paper you didn't even write? If they want to edit Wikipedia, they can do it themselves.
I'm tempering my urge to turn this into a full-on rant about the state of education in America, why no one learns to write well, that students aren't taught the value of being able to synthesize information, how you and your friends could possibly have gotten to your junior year of college with this kind of work ethic. I could go on. This isn't my usual "consider all the options and do what feels right," type of post because the only thing that's right is for you to do your own work. I'll leave it up to you whether you want to talk to your friends about what they're doing or even turn them in. I'm pretty sure you won't do either, and it's hard to blame you for that given the pressure you're experiencing on a variety of fronts.
I'll leave you with this thought: There's a quote in Pirkei Avot chapter 6:6 that says, "One who says something in the name of its speaker brings redemption to the world." Essentially, this teaching says that citing your source helps to bring about the coming of the Messiah. (With thanks to "Ask the Rabbi" at chabad.org for helping me find the source!) That's how much Judaism values academic honesty. If you place even a fraction of that value on the integrity of your work and your college experience, I'm sure you'll do the right thing and go to the library without your friends, write your paper from scratch using your own thoughts and include a citation each and every time you use someone else's words or ideas.