A data directory just points you to the locations of the data. Think of a Web search engine as a data directory. You type in the data you are looking for, and it tells you where it is at.
A data dictionary is information about the data, but not just limited to the location of the data. For instance, the data dictionary in an Oracle database can tell you what columns are in a table, what datatypes those columns are, who has permissions to view that data, and how that data in that table is related to data in other tables.
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This was first published in April 2003