I want to keep the products in Add to cart for guest user when He leave the site how is it will be possible?? for example I'm a guest user n I add few products in Add to cart n then I close the browser or the site but when I reopen the site then the products which I add before already exists in Cart how is it will be possible??
Magento does this automatically. You can adjust the cookie lifetime in the backend, which will determine how long the cart is stored - I think default is 1 hour, you can set it to 2 weeks or what you like.
When the user deletes their cookies, though, there's no way to have him keep cart items without a customer account. This is because the PHP session has, without a cookie, no way to determine who this user was.