3 ? tier : It contains client, webs server and data base 
server.
         2 ? tier:   It contains client and database server.

2 tier architecture : This has 2 parts namely client (front
end) and database (back end).The window application
generally developed in this technology 

3 tier architecture :
1 Client-front end (request is given through client)
2.business logic :Middle layer (protocols are used  for
processing the request)
3. Database :back end (response is given to the client)
Web applications are good examples for 3 tier architecture

two type of 2tier architecture
1.fat Client
      here the application or business layer is associated
with user or presentation layer and data storage layer is
reside separately.
Ex.- by submitting user name and password before login into
home page 

2.fat Server
      here the application or business layer is associated
with data storage layer and user or presentation layer is
reside separately.here all function is carried out on the
server.



3tier architecture
      here all 3 layers such presentation layer, application
layer and data storage layer is residing separately either
on same machine or on the different machine.
Ex-client submitting request in presentation layer and send
it to business layer for validation which in turn interact
with data base and retrieving the requested information from
it and send it to the client.