Plagiarism is using the words and ideas of another person as your own. It's often done accidentally, but whether or not it's done purposely, it's still wrong. The most common form of plagiarism is being careless about correctly citing sources -- or not doing it at all. Plagiariam is taken seriously in the academic world. It can ruin professional reputations, earn a failing grade, or even be cause for expulsion from college.
But it's also easy to avoid, if you know how. The links on this page can help.