When you upload a document to TurnItIn, the program checks your document for originality by comparing it to BILLIONS of other documents, including websites, journals, and even papers previously submitted to TurnItIn. The program then generates an Originality Report that shows your professor which portions of your paper match content in other sources. If your professor allows, you can also view the Originality Report and use it to ensure that you did not accidentally plagiarize.
View the graphic below to get a sense of the report's format and content.
Any matches to sources of Turnitin content will appear as color-coded, numbered passages.
Watch this video to learn even more about the Originality Report.