Client-server architectures are called as two-tier 
architectures. In the two-tier architecture, the user 
interface runs on the client and the database is stored on 
the server. The actual application logic can run on either 
the client or the server. 

Web architectures are called as three-tier architectures. 
In the three-tier architecture, the user interface runs on 
the client and the database is stored on the server. Third 
tier is for the application logic. The third tier is 
between the user interface (client) and the data management 
(server) components. This middle tier provides process 
management where business logic and rules are executed and 
can accommodate hundreds of users (as compared to only 100 
users with the two tier architecture) by providing 
functions such as queuing, application execution, and 
database staging.