The free allows you to publish and sell through there site and the paid packages allow you to sell through booksellers world wide, including Amazon, Barnes and Noble, waterstones etc.
I started out with the free version but soon went up to the mid package and I have to say how I was pleased I am with their service and with the final product once my book landed on the doormat.
My book is now featured on the above mentioned sites and I have recently signed up with Amazons KDP program, of too which I am very pleased as and my book sales, are slowly but surly increasing. And promoting my product has certainly helped, by using the various social media platforms, I feel Twitter has given me the greatest following, although I do have an author page on Facebook, of which I advise anyone to do and in particular get an author page on Amazon if you use it, as this will give your fan base more info on you as an author. As does my webpage and blog site. I have also joined varying communities for self promotion and use some paid services to promote my work too. And off of the back of this, I now have a literary agent currently reading my book, so hopefully if she feels I have something and everything crossed we can work together to further expand my presence. I will keep you posted on how this goes.