Can,
While these are not the "official" definitions from PTC, here's my understanding of the permissions, using a WTDocument as the item it is applied to as an example:
Full Control: combination of all the permissions below.
Read: see the object in Windchill
Download: if the object has content (primary or attachments), download a copy of the content to your computer
Modify: check out, edit attributes, and check in the object
Modify Content: upload additional, remove, or change the uploaded content
Modify Identity: change attributes associated to the master object (for WTDocument this would be name and number - this essentially allows the "Rename" command)
Create By Move: create a copy of the object using the "Save As" function
Create: make a brand new instance of the object
Set State: use the "Set State" action to set the life cycle state directly on the object (different from a promotion or change)
Revise: Update the revision to the next value in the series (or to any future value, depending on preferences)
New View Version: not commonly used in my experience, but you can create a new view of this object. I believe this applies more to WTParts having either a design view or a manufacturing view
Change Domain: again not commonly used in my experience, but this allows you to change the set of policies that govern the object (such as ACLs) without changing other attributes on the object.
Change Context: move the object to a different product/library
Change Permissions: allow the ad-hoc granting of abilities particular to this object (note that you cannot "ad-hoc deny", or overwrite an ACL deny)
Delete: completely remove the object from WC
Administrative: also not commonly used in my experience, I believe this allows the user to perform actions on the object that typically only Org/Site administrators can do ... possibly such as "Reassign Life Cycle" may be one of them.
Implementing ACLs can get a bit complex, so if you're looking for some advice/help on implementing ACLs in your system I'd be happy to help you out!