online library ? How do they work ?

I heard on a news channel something about online libraries ? do they exist how do they work ? I also heard something about Google trying to create the biggest online Library.
I am talking about free online libraries where i suppose you must be able to download the Book, or whatever material you want

I dont really get it but do you mean that you can use computers in the libary if you do ya you can. But if you mean you can shop for books on a online libary ya you can.