Turnitin is a database of papers written by students (in HS and College I know for sure), as well as other works accessible on the internet. A selling point for the company is that they have such a vast database. If you write a paper for a class, and turn it in to your professor, they then submit it to Turnitin (or you might do it yourself). The program scans your paper, and gives you a score based on the words and phrases you wrote. The technology is similar to that of Google. It reads the words typed, and matches it with other words and phrases in either identical order, or a very close approximation. The higher the score, the more your paper matches other works in their database. A score of 100% means your paper is a straight up copy of another. The lower your score, the less your paper resembles anything in their database. The works that match yours will have a link, and references, so you can see what your paper is matched against. I personally usually score in the low teens, but have gotten as high as a 30% (on an opinion paper, with no quotes and no references of all things!)- so bear in mind that even if you are honest you may still have some matches. Each professor or school may have different standards; some say anything more than 35% is a copy, others may go higher or lower.
And if you think about the English language,the recurrence of words and phrases in certain order and the fact that humans are all basically creatures of habit, odds are strongly in favor of matches. As another poster already said, medical papers (and any other specialty I would say) will have a higher rating of being similar, as there are only so many ways to describe the etiology and physiology of coagulation or wound healing for example.
My personal objection is that every time I submit a paper to Turnitin, it builds their database. Okay, so maybe 1-5 papers per class each semster might not seem like a big deal, but I worked on those papers, did the research, did the writing and editing and as far as I'm concerned they are my intellectual property. I have never signed any releases allowing my personal property to be used for their financial gain. Each semester I sign a release to allow photographs of myslef to be taken and used for the college brochures -never happened, not model material I guess but nothing for Turnitin. Now, as a student at a school that uses Turnitin, my very paying of tuition and agreeing to abide by the terms of the college may have negated any rights I have in that regard. I have yet to see any such documentation and specific rules or regs mentioning this type of technology. I have read my handbook cover to cover, and asked various admin. personnel, and still cannot get a clear answer on my rights. Obviously, if I objected so strongly, I could decline to utilize Turnitin, but I then run the risk and consequences of failing the course or being dropped from the program for not completing an assignment.
Of course, this whole thing maks me hate cheaters even more. Thanks jerks, for creating an entire industry designed to catch your sorry selves red-handed.
If anyone has a better description, or feels differently than I, I'd love to hear it. I've been disgusted about this for years, and am open to having my mind changed.