Online Booking Systems are websites or other "interfaces" to a shared database. The database holds all of the information available to all users. Any changes to it are also shown to all users.
A popular application of online booking systems is for booking flights. Users can visit an airline web site and click on a link to show available flights. This runs a query on the central database:
The results of the query are then shown in a new web page. The user can book the flight directly by entering details in a form on the web page.