The Roster block shows a page with a list of users in a course, with either a simply list of names,email, etc.  (similar to the participant functionality), or a more enhanced list that shows their picture and the description from the student profiles. This is useful for faculty to get to know their students quickly and learn about their backgrounds easily.

The latest versions (for 2.4, 2.6 and 2.7) fix a bug where the settings page on sites installed from Moodle 2.x did not show the list of roles properly. (This worked on 1.x => 2.x upgraded sites).

Additionally, in version for 2.6+, you can now set not showing the 'Last Login' information in several of the views.

The image size on the report can be chosen, so you can format as you wish.