The companies that are providing the service you are using must spend money to keep their servers running. These servers are what allow other people to look at your webpages. It is only fair that you allow the service you are using to place a small advertisement somewhere on your pages. Otherwise you can spend the money and get your own servers, domain name, etc., which is very expensive. I also have to say that this trick may not work with every service out there.
Different services use different methods to advertise on their users' pages.
I cannot guarantee that this trick will work with all of the free services out there on the 'net. Okay, enough of that. It is actually very simple to remove the banners. It only requires the use of a little bit of style sheet coding. I encourage you to check out the Cascading Style Sheet guide to get a full understanding of style sheets.