With Address Book Server you can synchronise your Address Book with others on the same network as well as over the internet. Contacts are synchronised with your own instance of Address Book Server.
Address Book Server comes in two parts a server and a client. The server should be installed on a centrally accessible Mac. The client goes on every Mac, whose Address Book you would like to synchronise.
During synchronisation contact information is fetched from the ABS server, compared with the local content, and any changes are written back to the server. Changes made via Address Book are synchronised in real-time with the server. Other clients will be brought up-to-date during their next synchronisation. This can either be done by making a change to one of their contacts, manually via System Preferences or on a schedule.