Extra credit grade items are not included in the overall course total, but can help raise a student's individual score. You can either create a manual grade item and make it extra credit, or designate an existing activity (Assignment, Quiz, Forum, etc.) as extra credit in the gradebook. Note: If you are going to have multiple extra credit grade items in your course, you might consider creating a cateogry and making the category extra credit instead of making each individual item extra credit.
If you do have any extra credit items we recommend showing the Calculated Weight (Weightings) column in the User Report as this displays to students which items are extra credit. By default that column is visible, so as long as you haven't hidden the column students will see it.