Plagiarism

Plagiarism is the use of others' material without proper attribution. There are very few places where it's accepted, though creationists and alties, among others, are notorious for plagiarizing from each other. Self-plagiarism (i.e. quoting yourself without pointing out that you've said it before) is seldom accepted either, though some people have made a career out of it.

Plagiarism is often considered a form of stealing (though others argue it's simply a form of copying, not a zero-sum act like theft) and, when caught, makes the plagiarizer look lazy and incompetent. Entire careers have  over the practice. Please, please, please don't do it, even if your original material is in the public domain (And if you must, be sure always to call it please "research").

Also, having your boyfriend/girlfriend edit your paper for you (without double-checking what they did) is plagiarism too. Doubly so when the professor finds out that two pages of what they added for you are ripped directly from encyclopedias available online. So is submitting the same paper for two different classes. Don't try it. People can TA more than one course. Also, just because you never used a source that contains the exact same wording as yours doesn't make what you did original; it just means you and someone else both plagiarized the same person. Seriously – professors are not stupid.

Copyright
Plagiarism and copyright are orthogonal. Plagiarism is about credit, copyright is about whether you're allowed to copy at all (“copyleft” licenses, like Creative Commons and the GNU Free Documentation License, exist specifically to restrict plagiarism without invoking copyright). There are no laws preventing people from plagiarizing or even completely copying works in the public domain (i.e. works that are not copyrighted), such as work by the United States government, formerly copyrighted works whose copyright expired, or works deliberately put into the public domain by whoever would have held the copyright, but giving credit is still a good idea due to ethical reasons (as opposed to the usual combination of ethical and legal reasons).

(Unless you live in a country which recognises as part of its copyright law, or has doctrines such as . Then, especially where the former applies, it goes against the law as well.)

How to
This looks like a great place to start!

On the other hand, in order to avoid getting a failing grade for plagiarism, one can consult this website, or better yet, just do your own work.