Turnitin is an online software program that can detect similarities between an existing work and your submittedwork. When you submit a work to Turnitin, the proprietary software compares that work to Turnitin's database.
Similarities are instances when your words match words found in other works found in the Turnitin database. Turnitin's database contains billions of works, such as:
After the Turnitin software has completed comparing your work to the contents of its database, a Similarity Report is generated. This is also sometimes known as an Originality Report. They are the same thing. For more information on the Similarity Report, click here
Turnitin detects similarities, and not necessarily plagiarism.
Plagiarism is using someone else's words or ideas and passing them off as your own.
When your work is run through Turnitin, any significant similarity to another work will be identified. This means, if you are using quotes and citations in your work (which you probably are), they will show up in your Similarity Report. This does not necessarily mean that you have committed plagiarism. Your instructor will determine whether the similarities in your work are actually due to plagiarism.
Source: University of West Florida Libguides