D2L is an online course management system. It is a collection of web based applications or tools seamlessly bundled together for the purpose of teaching and learning online. No instructor uses all the tools available in D2L; it is expected that some will be ignored.
Each new course in D2L has a course shell. This shell includes a navigation bar at the top (Nav bar) which includes links to the main tools. In most cases, the Nav bar is always visible at the top of the page no matter which tool is active. Whenever you get lost, clicking My Home always takes you back to your D2L homepage with your list of courses, etc. Also, Course Home always takes you back to the homepage of the course you are currently in.
Clicking on Classlist produces a listing of everyone in your class, including all instructors and all students. Typically, the instructor and student lists are regularly and automatically updated based on course rosters in the Student Information System (SIS).
Other commonly used tools in D2L include Content, Dropbox, Discussions, and Grades. Each of these will initially be empty until something is created or imported.