2 tier architecture is nothing but client and server

Means It consits of client(Presentation layer) and server
(may be database)
Client gives the requests through the browser for HTMl 
pages stored in remoting machine(Server).The server locates 
this files and passes to the Client
Only presentation layer and database server exits

Request are in the form of Http or FTP
Most of the actions done on the client side so client is 
thick and server is thin



Web application consits of Presentation layer( client or 
browser)Business process and database layer

As i mentioned earlier
Presentaion layer is useful for to submit the requests in 
HTTP form

Web server recieves the Request in HTTP form and sends to 
application server. Application server process the Request 
and gets the information from the database and sends back 
to Web server. WEb server sends the recived data in the 
form of HTML to the client.

Most of the action done on the server side
so client is thin

Presentaion laye<-->web server<-->application server<-->data