Plagiarism is the improper borrowing of another person's words, ideas, or methods.
If you use another person's material, you must acknowledge your source. When you cite a source properly, you have given credit where it is due, and you have also given your readers a way to locate the original material on their own. Also, plagiarism undermines the work you are doing as a student. You are not expected to have ideas that consistently rival those of experts and professional scholars, but you are expected to know how to find expert opinions on a given subject and how to properly cite those opinions. With the accessibility of the Internet comes new temptations to plagiarize.