An entry is a folder, document, shortcut.  An entry link is a link between two entries.

The labels define the two parts of the relationship, like "message" and "attachment".

Entry link type is the kind of relationship it is, like message/attachment or parent/child.  I think we create 1 or 2 types, but for the most part you would define your own.

Document is a utility class to get a DocumentInfo object, which represents a document.  The DocumentInfo may have links associated with it, and each link has an associated type.

Documentation is included in the SDK installation.