1. Client server applications run in a networked 
environment,and although you have two separate components 
to test ,you still have control over the client part of the 
application.

   Web applications run in a networked environment but you 
loose control over the client. Web applications are 
supposed to run on various browsers and platforms.

2. Client server software must be tested at the native 
operating system level.
    Web software gets tested higher up in the food 
chain,far away from the bowls of the operating system.

3. Instead web server software must be tested for browser 
compatibility and operating system compatibility. But also 
because so many various programming languages and scripts 
can live with in a single web page,the interaction of 
various applications must be also be tested. External  
software,such as shockwave and javascript and java and perl 
along with network connection must work together to create 
the expected result.