Again, a big portion of its job is to format and display the application interface, but it also is responsible for crunching some numbers. In a fat-client application, the server portion of the application retrieves all the appropriate numbers for each senator's office and returns them to the client. The client then adds up the numbers to get a total for the whole Senate and calculates each senator's use as a percentage of the total, perhaps throwing in a chart for good measure. In an n-tier architecture, the work is split up even further. In addition to a display tier and a server tier, there can be an application server.
Web applications use Web browsers as the client.
Because of this, Web applications are almost always thin-client applications. The exception is when applets are downloaded to a browser, introducing some fat-client processing into the application. Browsers render whatever they receive from the server, so if you need to change the interface for an application, you need only modify the central application's interface functions. In contrast, updating a classic client-server application for which a programmer wrote a proprietary interface involves going to every desktop PC and loading new software with the new interface instructions.