Give Credit Where Credit's Due
Plagiarism—the attempt to pass off the ideas, research, theories, or words of others as one's own—is a serious academic offense. Most students know when they are intentionally plagiarizing, for example copying an entire essay out of a book or buying a paper off the Internet. However, many people are tripped up by unintentional plagiarism—not giving proper credit for others' quotes, facts, ideas, or data.
When in Doubt, Give Credit
A good rule of thumb is to always give credit for any ideas that aren't yours by citing your sources. Different subjects and teachers have different standards for citation. Usually, your teacher will specify how you should present your citations, and if they don't, ask.