Turnitin is a service that reviews student papers, compares them against documents in its database (which includes websites, journals, papers submitted by students across the country, etc.), and generates a report indicating how much of the paper matches text from these sources. 

When you submit an assignment to Turnitin the first time, you will be asked to authorize an End User License Agreement (EULA). You can review those terms and conditions prior to agreeing to them. Regardless of whether you accept or decline the EULA or choose not to respond either way, it is possible for you to make a submission. If you decline to accept or skip the choice, your submission will not be submitted for review by Turnitin. According to US Department of Education rules, instructors have the right to require students to submit their work to an originality verification tool such as Turnitin. Thus, submission to Turnitin may be necessary in order to receive credit for an assignment, at your instructor’s sole discretion.

Learn more about submitting an assignment or forum post to Turnitin.