Often plagiarism is unintentional, resulting from a lack of understanding of how to honestly and accurately give credit to the sources you use.
Plagiarism = using someone else's ideas, words, photos, music, and so forth without giving them credit for creating.
Citations = how we give credit to someone for creating ideas, words, and the like in our own work.
Whenever you summarize, paraphrase, or quote someone else's work without acknowledging the source, you have fallen into plagiarism. Many times students unintentionally plagiarize. Please see the tabs on Common Misunderstandings about Plagiarism and Tips to Avoid Plagiarism to help you figure this out.