At this time of year, we hear a lot of chatter from students via Twitter (follow us: @turnitin) saying, "turnitin says my essay was 23% plagiarized," or "just submitted my paper to turnitin... 4% plagiarism is good right?," or "my Turnitin plagiarism percentage is only 18%."
There is a very distinct difference between what Turnitin flags as matching text (aka: similarity index) and plagiarism. Turnitin will highlight ANY matching material in a paper—even if it is properly quoted and cited. Just because it appears as unoriginal does not mean it is plagiarized; it just means that the material matches something in the Turnitin databases.
We leave it to the instructors to look at a paper and the originality report to make the determination of whether or not something is plagiarism, and to what extent—intentional plagiarism, unintentional plagiarism, improper/lack of citation, or mere coincidence. Best practices from instructors suggest that Turnitin OriginalityCheck be used as a teaching tool to address citation and academic honesty, not only as a punitive tool.
There is also a helpful video for instructors on how to view the OriginalityCheck Report and goes over many of the OriginalityCheck features. There is a similar video tutorial for students on how to view their OriginalityCheck feedback as well.