I have developed <snip> website. Now i plan to change design into Bootstrap CSS. so i am not much knowledge about bootstrap design. so anyone could explain about bootstrap CSS and also explain advantage and disadvantage.
It's a "framework", meaning a bunch of CSS and HTML already written for you. If that's what floats your boat, then go for it, but IMHO frameworks are more trouble than they are worth.
But your question is too general. Do some reading here:
and then come back with some specific questions if there's something you don't understand.