Smashwords is a publishing platform. It uses the Meatgrinder to create copies of your files to distribute to all platforms.
See also D2D (Draft to Digital)
Smashwords and Amazon Kindle Direct Publishing are the oldest of the self-publishing sites. While Smashwords distributes to many sites, and has the widest variety of options for files, it is also not as popular as going direct to each platform, or Amazon, due to the issues with the ‘Meatgrinder’.