compatibility testing particularly in web applications

1.what all browsers are compatible.
2.what all versions are compatible.
3.GUI plays vital important.
4.Performance also.

Compatibility testing is nothing but
checking the compatibility between browsers, operating 
systems.

To check whether our application supports customer expected 
platforms or not?

Checking the application on different platforms (Operating 
System) with different browsers,the main aim of 
compatibility testing is to test weather application can 
run successfully on different OS.

Test the application between different browsers with 
different versions,and different OS (like windows 
9x/xp/2000..,linux),different Dbs,and different networks

Compatability Testing: Testers will run the app on the user 
required platfrom to verify whether the app is compatable 
or not.

Configuration Testing:  Tester will run the app on diferent 
platfroms .

Test the application between different browsers with 
different versions.
For example browsers like: 1) test the application on      
       1)"IE 6.0"
2)"IE 7.0"
3)Mozilla firefox
4)Net scape(Within the netscape again there are two types
they are a)thru IE
    b)Firefox