BBDB is the Big Brother's Insidious Database, a package for Emacs originally written by Jamie Zawinski which provides rolodex-like database functionality featuring tight integration with the Emacs mail and news readers.
It is often used as an enhanced email address book.
EUDC considers BBDB as a directory server back end just like LDAP or PH/QI servers, though BBDB has no client/server protocol and thus always resides locally on your machine. The point in this is not to offer an alternate way to query your BBDB database (BBDB itself provides much more flexible ways to do that), but rather to offer an interface to your local directory that is consistent with the interface to external directories (LDAP, PH/QI). This is particularly interesting when performing queries on multiple servers.
EUDC also offers a means to insert results from directory queries into your own local BBDB (see Creating BBDB Records)