There are many ways to design client server applications, using levels called
tiers to organise the application:
single tier. Here a mainframe talks directly to a dumb or block mode terminal.
The mainframe is in complete control with a monolithic application.
two tier. Here a database engine on the server talks directly to a fat client
(e.g. Applet or application).
three tier. You use a thinner client and a middle layer containing the business
logic. In a three tier webserver the middle layer fetches data from the database
and converts it to HTML (Hypertext Markup Language) to be
rendered by the browser. The advantage is middle layer is made of more reusable
code and it isolates changes in the client and changes in the database from each