What is this thing that is called a "Data Dictionary?"

Many of the ERD and SQL books that have have read refer to a data dictionary.  However, I seem to
be missing the point of what it acuatally is.  IIRC, some books refer to it as a kind of document
created by a DB designer, and other suggest that it is a kind of RBDMS created metadata.

Can anyone please offer a consise description or provide a link to that describes what it is?
Also, how is this information useful to a DBA?

Thanks for the help!

Regards,
Richard Broersma Jr.