Server side testing is valuing the clent parameters after 
reaching to server, where as clent side testing will do 
that client machine only.

Server is testing will be done thru application programming 
languages like java/jsp, where as client side testing has 
been taken care of Java Script.

Server side testing is required in below cases:
1: Required values from database to process/valuate the 
Client parameters.
2: Required to apply the business logic.
3: Required any type server side resources to check user 
resuorces.

Server side testing is required in below cases:
1: validating parameters.

client side testing means when you will click on submit 
button that time request will go in the database and it 
will check your request is valide or not if it is valid 
then it will send response back.

server side testing means when you will click on submit 
button that time request will go server side and if will 
check in the validation method in case of java script and 
validation / execute method in case of struts, if it is 
valid then it will response back to the browser.