This screen will show you who has accessed the class in order of most recent first. From here you can choose several other view option.
Here you can choose to see users by group or inactivity.
You may also click on a participants name, then click on Activity Report tab to see what activities a participant has been active in.
Then:
The Outline report simply shows which activities a participant has accessed. The Complete Report will show forum posts and responses as well as what activities the participant has viewed.