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.

Projects are broadly divided into two types of : 
2 tier applications 
3 tier applications 


CLIENT / SERVER TESTING 
[a] This type of testing usually done for 2 tier 
applications (usually developed for LAN) 
Here we will be having frontend and backend. 

The application launched on frontend will be having forms 
and reports which will be monitoring and manipulating data 

Eg : applications developed in VB,VC++,Core 
Java,C,C++,D2K,PowerBuilder etc., 
The backend for these applications would be MS Access, SQL 
Server, oracle, sybase, mysql, quadbase 

The tests performed on these type of applications would be 
- user interface testing 
- manual support testing 
- Functionality testing 
- compatability testing & configuration testing 
- intersystems testing 

WEB TESTING 
[b] This is done for 3 tier applications (developed for 
Internet / intranet / xtranet) 

Here we will be having Browser, web server and DB server. 

The applications accessable in browser would be developed 
in HTML, DHTML, XML, JavaScript etc., 
(we can monitor thru these applications) 

Applications for the webserver would be developed in Adv 
Java, ASP, JSP, VBScript, JavaScript, Perl, ColdFusion, PHP 
etc., 
(all the manipulations are done on the web server with the 
help of these programs developed) 

The DBserver would be having oracle, sql server, sybase, 
mysql etc., 
(all data is stored in the database available on the DB 
server) 


The tests performed on these type of applications would be 
- user interface testing 
- Functionality testing 
- security testing 
- browser compatability testing 
- load / stress testing 
- interoperability testing/intersystems testing 
- storage and data volume testing

Hi All,
Let it simplify a bit
2-tier application is the client -server aplication with 
frontend and backend.
At frontend there would be forms, reports which gets filled 
and monitored by backend.

3-tier application has 
presentation layer (browser)
Application layer (code)
Database layer (Database-tables,views,procedures )
web application comes under 3 -tier and window based or 
desktop aplication falls under 2-tier.
Suggestions are welcome