Gagan,
init() method is the way to put initialization code to your servlet. For example if you wanna open a DB connection or initialize any kind of code, init() is the place to do it.
On the other side, there is destroy() method that can UNDO everything you have done in init( close connections or similar ).
If you're a newcomer( I believe you are

), welcome to the servlet world.
I would recommend
this free book for learning servlet. It is for the previous version of servlets, but it is a start. I would recommend to buy the second edition if you have the funds.
Happy Learning!