Smashwords is both a retailer and a distributor. They will sell your book when you upload it, but they will also allow you to distribute it to many other retailers who will then sell it on their sites, too. Smashwords' authors can opt out of this distribution on an individual retailer basis, so if an author already has his book published via Amazon's KDP platform, he can opt out of shipping it to Amazon.
From everything I have heard, the Smashwords feed to Amazon isn't actually implemented yet. I recommend loading a book directly to KDP. For other retailers, though, Smashwords is a good bet, so long as you can live with really slow feedback on sales. KDP shows new stats within hours of a sale (usually); Smashwords is the same for sales from their own store, but they can take more than a month to update sales data for a specific retailer.
When you opt in to distribution, Smashwords will deliver an epub file (except for Amazon!) to that retailer, but they will refer to it as shipping the book. I find that weird!It's not like you pack a digital file into a padded envelope or a cardboard box!
So far, Where Magic Rules has shipped to Barnes & Noble and to iBooks, but it's not showing up in either store yet.