two tier archtecture is client server and three tier 
architecture is web based application.

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.                                                    
       M  Dora Babu   S O N T A S O F T W A R E     
BANGLORE     Mail me @  mdorababum@gmail.com

hi i m swapnil
Two-tier architecture is a client/server architecture , 
where a request to do some task is sent to the server and 
the server responds by performing the task.where as a three-
tier or a multi tier architecture  has client, server and 
database. Where the client request is sent to the server 
and the server in turn sends the request to the database. 
The database sends back the information/data required to 
the server which inturn sends it to the client.

two tier means two layer available.three tier means three
layer available.

In Two-Tier Architecture we have just two layers 
1. GUI/ intrface / presentation layar
2. Database layer (where we store/retrieve our data) 
Basically this architecture is followed in client/server 
applications where database and application resides in the 
same machine or database may in other machine conected 
thtough network. 

Three-Tire Architecture: 
In three tier architecture we can find three layers names:
1. Presentation Layer
2. Busness Layer/ Mid Layer 
3. Database Layer.
 
Presentation Layer: GUI/Interface , where a user view or 
enters the data.

Business Layer: where business login and rules are execute 
and can accommodate hundreds of users.

Database Layer: Data access layer takes care of connection 
between the appication and storing the data as a warehouse.
This is very important layer in any architecture 'coz 
finally all data comes to it and maintaining such a huge 
amount of data is taken care by this layer avoiding 
concurrency and maintaining scalibility, durability to the 
application.

Two tier architecture : 

        Client server application. Client acts as Front End 
and Server acts as Back end (databases).
       Here where client make a request to the server where
the query evaluated in the server and returns the request
given by client .where here all the query evaluated in the
server side  .

Three tier Architecture :
Three tier architecture means N tier architecture
1. Presentation Layer
2. Business Layer/ Middle Layer(tier) 
3. Database Layer.

  where here client make a request to server through
presentation layer  to get some data the query is evaluated
in business layer and the data is retrieved from the
database layer(all the complicated work done in middle layer
) . Database layer is responsible for all the connections
between the server .so the accessing through database make
easier through 3tier architecture