The Oxford English Dictionary explains that to plagiarize is "to take and use as one's own thoughts, writings, or inventions of another." Its definition of 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 Code of Academic Conduct defines plagiarism as the “Unacknowledged or falsely acknowledged presentation of another person's ideas, expressions, or original research as one's own work. Such use is defined as plagiarism regardless of the intent of the student.” In effect, all definitions of plagiarism suggest that using, borrowing, appropriating another's work (words or ideas) in any way without acknowledgement or proper citation constitutes an instance of plagiarism. It is the language equivalent of stealing from another whether it involves using another's exact words without giving credit or paraphrasing those ideas, opinions, graphs, statistics, facts, etc. (that are not common knowledge).
Turnitin.com is a resource utilized by Tulane University where students submit their work and it is checked against a database of previously submitted works to check for signs of plagiarism.
They have a helpful Plagiarism Spectrum page that explains more about the different types of plagiarism. Here's a summary of the types of plagiarism and level of severity of each: