Plagiarism means presenting other people's ideas as your own. It is a serious offense against academic and personal integrity that may result in a failing grade or expulsion.
Not all students understand plagiarism, and some are surprised when they are accused of committing it. The mistake made most often is failure to realize that any borrowed material should be documented, even if it is very familiar to you. Your course textbook, for example, should be cited properly when you quote or summarize from it in a research paper. And, even if you change the words of the text to your own (called paraphrasing), you still are required to provide proper citations. Ignorance about plagiarism does not constitute an excuse. You plagiarize whenever you present another's ideas or words as your own, either intentionally or by mistake.
Plagiarism represents a disconnected kind of thinking characterized by patching the works of others together without a coherent thesis. If you start with your own research question, you then easily find ways to use your sources to support your point of view. The payoff for academic honesty in this case is a paper that others enjoy reading and that expresses your original thoughts. Once you begin with your research inquiry, then, plagiarism becomes a technical problem, one that is solvable by simply learning the rules for documenting research sources and when to apply them.