Desktop application runs on personal computers and work 
stations, so when you test the desktop application you are 
focusing on a specific environment. You will test complete 
application broadly in categories like GUI, functionality, 
Load, and backend i.e. DB.
Web application is a bit different and complex to test as 
tester don’t have that much control over the application. 
Application is loaded on the server whose location may or 
may not be known and no exe is installed on the client 
machine, you have to test it on different web browsers. Web 
applications are supposed to be tested on different 
browsers and OS platforms so broadly Web application is 
tested mainly for browser compatibility and operating 
system compatibility, error handling, static pages, backend 
testing and load testing.