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