Can anyone explain me about gestures? What are their use? Can u tell me any practical ideas where we can implement them?
There's an article on d.android.com on precisely this topic:
http://developer.android.com/training/gestures/index.html
Gestures will recognize an arbitrary pattern drawn by a user on the screen, and then allow your application to perform an action as a result. You use them whenever you think this behavior would be more intuitive than a button or to save screen space.
As for practical ideas, it really depends on the nature of your application.