Microsoft released a pack of Dynamics AX entity relationship diagrams. This pack covers most core tables of the product. For example an ERD of Customer table
These diagrams are available in http://www.microsoft.com/dynamics/ax/erd/ax2012r2/Default.htm
To help you find the correct information you also have an Alphabetical list of tables.
You can also build by yourself these diagrams using the built in Reverse engineering tool, please check http://msdn.microsoft.com/en-us/library/aa499193.aspx