Web Applications,in general use 2 tier which is a client/server Architecture.But to have more scalability they use 3 tier architecture.

2-tier:





2- Tier consist of two separate applications, one server application and a client application.





3-Tier:


It consists of
1 Client-front end in which request is given through client
2.business logic :Middle layer in which protocols are used for processing the request
3.Database :Back end database server like Oracle, MSSQL or any other standalone database)

Here the middle tier is also called Application server which communicate between web browser and database on another system.
Web applications are good examples for 3 tier architecture