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.