It is a two tier application .For the accesibility of the
application by many users simultaneously the client machine
interacts with the common dataabase
(databaseserver)installed on a machine.

Disadvantage: Any change in the application program should
make the changes in the each application installed
seperately on each machine.

so in order to overcome this we have a 3 tier Architecture
known as  web based application.
Web server acts as a middle tier beetween the client machine
and the DB server.
The main source of changes in the application program are
done in the web server taking the neseccary input from the
client machine.

For ex: The change in the application is from adding two
no's to substraction of the no's . The client machine takes
the input of no's but the substractiion is done on the
server machine.