Hi Kaushik,
As everyone explained, smoothing groups is a good way in game art to give a idea for smooth surface in a low poly model.Here is a tutorial i found which explains in detail how smoothing groups work and can be used in the workflow.Credits to the author of the tutorial, Ben Mathis.Check out his
site for other good tips and tricks on game art.
http://www.poopinmymouth.com/process...ing_groups.jpg
May be next time, before posting if you could prepare a image showing the type of problem you are facing.As we are all artists here, putting some kind of visual/image is a good step towards getting a more clearer answer.
Hope it helps.