Account Administrators are people who can initiate projects on behalf of your company. If you will be creating projects in the future, you can be added as an account admin as well. Only another account admin can invite new ones.
Data Reference, or DREF, is a type of information link specific to Civil 3D data.
Desktop Connector is the Autodesk application that relays information between your operating system and Autodesk Docs. This is the application that adds the Autodesk Docs shortcut to the Windows Navigation pane and shows the 
next to your Autodesk Docs folders. For more information, see About Autodesk Desktop Connector.
External reference, or XREF, refers to any referenced object in Civil 3D. This includes DWG, DGN, raster, linked Excel files, PDFs, XML and so on.
Hub is a term found in Help files and discussion groups to refer to your company's main Autodesk Docs space online.
Project Administrators are the people who can make project-specific folders, invite collaborators and set permissions on portions of the project.
Role is one attribute that can be used to set permissions on a folder and/or grant access to various Autodesk Docs modules. The Project Administrator assigns roles.
Registered Applications or REGAPPS. Regapp is short for registered application. Using one of the Autodesk-provided APIs (AutoLISP, ObjectARX, .NET and so on) it is possible to attach Extended Entity Data (XData) to objects in a drawing. This XData allows different types of information to be associated with specific objects and extends data storage options in AutoCAD. XData is grouped by registered application name and a registered application name is required to identify the XData. In order for an application to store and retrieve XData, it must reference a unique regapp ID. A regapp name (or ID) is an identifier of XData that is (or was) attached to objects in the drawing.