Turnitin ("turn it in") is an originality checker that integrates with Canvas. Turnitin will scan written work and automatically detect if portions of student work have been plagiarized. Additionally, Turnitin can do grammar checks and grading. Turnitin also has an iPad app that can be used separately from Canvas.
Turnitin has been updated so that you can now turn it on directly in the settings of an assignment in Canvas. It no longer requries the "External Tool" option. You can also review these detailed instructions with screenshots.