Tier will be consider to as a layer in 2 Tier there is 
1 - Web
2 - Database 
main use for tier to provide fast response to client & and 
othere benefit to reduce load on server both(hardware & 
Software)

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.

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

In 2 tier applications client requests the server 
directly,there is no intermediate layer between them.

In 3 tier aplications there is middleware (Is nothing but a 
collection of diff. softwares),first client request is 
received by middleware & then it passed to server,after 
that server give response.

In Two Tier application if client hits the server then in 
Server the request goes, verified by the Databse or 
buisness logic as application layer and Database are 
connected in parallel and server replies to the client.
In Three Tier architecture when client hits the server then 
in server there is some temporary database connected in 
parallel with Application layer and after that the main 
Database server is connected with that server in series, 
after verification from the first database server
(Temporary) the data finally goes to second Database server
(Final repository).
This is how the architecture can be increased to n-tier.

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.

In other word you can say that in 3-tier architechure 
business logic,databases & presentation are maintained in 
different files whereas in 2-tier architechure business 
logic &databases are maintained in same layer or file
3-tier architechure in other word u can say fallows MVC 
architure

koyyala....the main difference is fake and real 
experience..first be wel prepare on the basis of practical 
oriented