Client server comunication with a centralized DB is one way to establish communication between client computers and some server, especially when the server is not directly reachable from the clients, but the database can be reached from the client side and the server side as well.
The mechanisms that can be used to notify either client or server of some incoming request or a response really depend on what RDBMS you are using. One of the easiest but "chattiest" methods is surely
polling[
^]. If your database is MSSQL 2005 you might want to look into
SQL Server Notification Services[
^].
Hope that helps you a bit.
Regards,
— Manfred