Tht's something even I use to wonder abt...but finally have an inkling abt the diff.
Computer Science :-
Computer Science is a discipline that involves the understanding and design of computers and computational processes.Deals indepth abt hardware/configurations and software and operating systs and all the techie crap!
Computer Application :-
Apply computer science and technology to solving problems outside the computer field-in education or medicine, for example. Gives u an overview abt computers and all tht goes with it i.e deals with all the above mentioned stuff with a barge pole and is more focussed on how to apply these stuff for practical purposes.
Though in my 3 yrs of study,hardly found anything tht dealt with the 'application' part.Studied abt OS,N/W,Languages,MIS,Web Prog,Hardware et al.It was all like introduction abt these topics and lil on the application part.
And one BIG diff......BCAs have more fun then their Comp sci peers!