This is a newbie question, but I didn't see any adequate information in the User Manual, and a search of these archives didn't make it clear either:  What do the various user access levels give permission to, exactly?  (by default)  I understand that these things can be (somewhat) modified, but there's no information (that I can find) on what they allow by default.  We're currently using 0.18.2, if that helps, in case the defaults have changed from version to version.
Also, if there's a listing of what can be modified in terms of access levels, that would be great.