The Grades tool allows you to enter grades for students, or link from other assessment tools in D2L (like Assignments and Quizzes) so that those grades automatically go from those tools into the Grades tool. You can add Bonus grade items, drop grade items, exempt students from grade items, etc. The Grades tool will automatically calculate your final grade for you, and you can export your Gradebook as an EXCEL spreadsheet to save to your computer. Students can see their grades as you add them, but do not see the Final Grade until you release it to them. Tutorials for students related to the Grades tool specifically are at the bottom of this page (and also on the D2L for Students page).
Here are some tutorials to get you started.