Whats is the Difference between Sql Server , DB2 and Oracle ?

well to start with they are basically from different companies....

Have you tried googling for the same... have you gone through their websites and features...which versions do you want to compare....

Your question is very open-ended.... for exactly what you want i would prefer you google on the same... if you have any specific issues then we can look into the same.

If you only look at the "main entrance", i.e. SQL and especially DML, the three have become more and more standards compliant during the last few years and hence more and more similar, in the sense of "SQL is exchangeable". (At least, if you write standard SQL.)
Implementation details will always be completely different, of course. (This even holds partly for the three different implementations of DB2.)

see also Comparison of different SQL implementations

Wow Saggi...you sure have a lot of homework this late in the semester, and some very basic questions that one would think were discussed at the beginning of the coursework, rather than this late in the game. Are you trying to prepare for a final exam or just rushing to make up for copious amounts of lager consumed during previous study hours?

Neither are necessarily bad, mind you...I'm just trying to get some perspective.