Plagiarism is "the wrongful appropriation or purloining, and publication as one's own, of the ideas, or the expression of the ideas (literary, artistic, musical, mechanical, etc.) of another."
The Oxford English Dictionary
So what does that mean?
Plagiarism is using someone else's ideas, words, or other original work without giving them credit. In essence, it is presenting their work as your own.
Sometimes it is obvious - taking someone's paper & putting your name on it - but sometimes it is more complicated. What does "original work" refer to? What about "common knowledge" - do I have to cite the source where I found that Toni Morrison was born on February 18th? This page provides links to many helpful guides to help you navigate these questions.
Taken from http://library.duke.edu/research/plagiarism/.
Plagiarism occurs when a student, with intent to deceive or with reckless disregard for proper scholarly procedures, presents any information, ideas or phrasing of another as if they were his/her own and/or does not give appropriate credit to the original source. Proper scholarly procedures require that all quoted material be identified by quotation marks or indentation on the page, and the source of information and ideas, if from another, must be identified and be attributed to that source. Students are responsible for learning proper scholarly procedures.