Assessment refers to work you submit and receive a grade for.
On your learnonline course site an assessment is generally identified with this icon or if it's a quiz, with this icon - be sure to check with your Course Coordinator to identify the assessments for your courses.
Assessment means the same thing as Assignment.
Turnitin is a system that checks your work against that of others to help you avoid plagiarism.
Gradebook is a term used sometimes to refer to assessments and marking generally.
Watch the video on how to submit an assignment and check out the Table of Contents on the left for more help on assessment.