The biggest d/f b/w Desktop and web application is-
  Desktop App (DA) is the machine independent, hence evry
change has only reflects at the machine level.
  Where as Web App (WA) is the Internet dependent program,
hence any change in the program reflects at every where,
where it becomes use.
  EX......
   Suppose there are 5 machines in DA, 5 time installed
individually at every machine and if there is any change
made in DA then at every machine change has to be made.
  In WA where the program or Application at the Server or at
the one common machine, then if changes made at only central
or server or common machine all the changes get reflected at
every client machine.

The major difference between desktop and web application is
that when desktop as name hints us that it is in stand alone
computer, when we talk abt software when downloaded in
desktop or in stand alone computer , that particular
software run on that desktop only . but in web appliaction 
the requirements is that networking should be there  and if
yes then just through URL we can access that web appliaction..

inmy view
desk top is  icon based interaction where we can go to view 
r enter by selecting icon.
web-- is a link based where conectivity is there through 
i/p or tcp to view world wide on same desk top

more over internet icon also present on desk top too but 
connectivity sholud be there to access.here authorisation 
and authen tication and accee con tro is there to any user
 may u pls can give more information

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.

1. We can create an shortcut(icon) over desktop in desktop  
application whereas we can not create a shortcut(icon) for 
web application.

2. If we make a change in desktop application then that 
change will not be appear on other application running on 
different m/c whereas changes will be display globally on 
web application.