Plagiarism is intellectual theft. You commit it whenever you submit a phrase, sentence, or idea that comes from some other source without explicitly acknowledging that source in your work. This is a serious academic integrity offense, and you should expect severe consequences if you commit it.
See IWU's student handbook and the University's official plagiarism statement. The consequences are severe. The penalty that I impose for plagiarism in my classes is failure for the course. (And no, you will not be permitted a "do-over" or a rewrite as if nothing had happened.) The university's policy is that I must notify the Associate Provost in writing of the details of your offense. If the Associate Provost has knowledge that you have plagiarized before, he will usually start proceedings to "separate" you from the university (this is a polite way of referring to expulsion).
Wherever you use phrases, sentences, or ideas that come from other people (classmates, instructors, books, web sites, etc.), you must attribute those views to a specific source. If you include a phrase or sentence that is not your own, you must indicate that the material is quoted. Enclose brief phrases or sentences in quotation marks. (Here's a decent rule of thumb when you're writing for me: if you take more than three consecutive words from a source, you should put the phrase in quotation marks.) Note: taking someone else's sentences and changing a word here and there or tweaking the structure a bit does not make those sentences your own.
Ask me! If you ask me whether something you've done counts as plagiarism before you've submitted it, no consequences will occur. But once you've handed the paper in, ignorance is not an excuse. If you are uncertain whether your paper is going to get you in trouble, the burden is on you to ask me whether your paper is acceptable.
Here are some web sites that discuss some examples of plagiarism and discuss the responsible use of sources.