Synchronization, as distinct from the above, is a stateful bi-directional operation. This page talks about the software, utilities and general issues related to keeping your BBDB in sync with other PIM (personal information management) systems.
ASynK? is a python program that performs bi-directional synchronization for Contacts, and works between MS Outlook, Google Contacts, and Emacs BBDB. The project page is at: http://karra-asynk.appspot.com/ As of March 2013 the software is under active development. Check out the project website for more information on finding the source, the asynk-users mailing list, announcements, and more.
Although the Palm handheld products are more or less dead now, they had their share of fame, and in its heyday, the Palm had multiple working bi-directional sync utilities for BBDB!
Be sure to search the BBDB Info mailing-list archives for the latest stuff.
CardDav? is a IETF standards-track protocol designed from the ground up for keeping PIM clients in sync with a contacts server. It has emerged as the most generic and open standard for contacts synchronization. Apple Addressbook can connect to a CardDav? server for synchronization. Gmail can act as a contacts server (you can think of the contacts UI in gmail as google’s CardDav? client). CardDav? is based on vCard format for storing individual contact information.