[gmw]

Amazon have KDP and KDP Select. If you choose KDP Select you must stay with Amazon exclusively for at least the period specified - read their terms. Aside from that, you can publish with multiple sellers and/or distributors. I currently use Smashwords, Amazon and Kobo.

Note that Smashwords is not just a reseller, it is also a distributor - sending your book on to many other stores and libraries (if you get into the premium catalogue). But if you have also gone with Amazon directly then you should tell Smashwords NOT to distribute to Amazon (for example).

ETA: Note also that (if you don't go exclusive with KDP Select) there are many other places you should look at, including: BookBaby and Draft2Digital (these two are also distributors like Smashwords - typically you would choose one of the three, otherwise it might get confusing about who you have distributing your book to where, since their distribution channels overlap significantly). There are others too.