Normally, auditors are restricted from participating in anything that gets a grade. So, no tests or assignments for auditors (they can, however, see that they exist).
Otherwise, auditors can participate in Bulletin Board, Discussions, Lessons, and Chat, as well as view the Roster (only the student view, of course!), Info, and Calendar.
If the course catalog permits auditors to submit assignments, then they can participate in assignments (and receive grades for them) just like enrolled students. However, their grade reports and transcripts will still display AUD in place of a grade.