Performance testing is basically done to check the effect 
of a particular level of stress or load on a application. 
For e.g. if an application is designed to handle a load of 
1000 users simultaneously, what would be the response of 
the application if number of users are increased to say 
1100 and so on.(load testing)
Similarly, if a application has been designed to work with 
a min configuration of 128mb of RAM we check wht is the 
response of the application working with 64 mb of ram 
(stress testing)
in the same way u can check ur database response if we 
increase the volume of data in it (volume test)
so performance testing can be done both online and offline

Hi


  Performance Testing is applicable for 
 1. Web Testing.

 2. Client Server.

 3. Desktop
   What ever the application may be but..
To check Response Time & through put .