You will have two download options:
1- An .epub file for upload to an Ipad/tablet, or for opening on pc/mac using an e-book reader, such as Caliper (http://calibre-ebook.com/), or Fbreader (http://fbreader.org/)
2- Alternatively you can choose to upload the .mobi file to your kindle device, or open it on your computer using Amazon's 'Kindle for PC or Mac' , or Mobi Pocket reader (www.mobipocket.com/)