This is the Kindle format, readable on all forms of Kindle – PC app, Kindle device, Kindle Android reader, etc. It is inferior to the epub format in that it does NOT support transparency or in line placement of images.
This is the format supported on certain major readers: Sony reader, Nook, Apple iPad, and Kobo reader. It is a clean, pretty format if formatted correctly. If uploading directly to Apple, you need an .epub that passes Epubcheck to perfection — and it’s very picky.
Smashwords requires a Word .doc formatted to their SPECIFIC requirements. You can’t just write a novel in Word and immediately upload it — it would be a nightmare. The Style Guide is 80 pages long and tells you step-by-step how to format for their conversion; if you don’t want to read it or attempt on your own, PLEASE, for the sake of the indie image, hire a professional :)
You can do a lot with your print books. Take a look at any trad pub in the bookstore, and you’ll see some of them are pretty fancy. Think you can’t do that? Sure you can. I CAN. The Createspace format can be created in any program, as long as you can print or save to PDF. I always recommend uploading a PDF to Createspace, and nothing less. This way, you ensure everything is right as rain and your fonts/margins/etc will remain where they belong.