Two tier architecture means: 
        Client server application. Client acts as Front End 
and Server acts as Backend like databases.

Three tier Architecture Means:  
     Three tier architecure means N tier architecture like 
Yahoo , gmail, and or Web Applications. Having Number of 
Users or Customers or 
Clients.

Three Tier Means client,Server and  more than one Business 
Layer.Each Business layer performs one opertaions.For 
Example yahoo, This have more business logic.It consist of 
Login,Check the correct user etc

two tire architecture is a client server application. 
client act as front end and srever acts as back end.
  Business layer and data base layer.



 THREE TIRE ARCHITECTURE is a web application.
Presentation
Business  and 
Data base

for ex  Presentation-JSP
Business-Java  and 
Data base- SQL or db or oracle

Tire architecure is introduced because of the follwing 
resons

1). To reduce the design complexity.
2).To make our system to withstand even the technology of 
particular module is changed i.e for eg:Data base
3).To support the interpropability.

 THREE TIRE ARCHITECTURE is a web application.
Presentation
Business  and 
Data base

If the business logic having more complexity then the 
business logic may have more than one TIRE.