I started out using my PC and the Amazon pc app. I picked up a bunch of the classics I read over the years for free and although many came from the Amazon store and were on the Amazon server (cloud) I downloaded them to my pc because I saw no value for the cloud.
Then I got a free Kindle E-ink reader with points I had from a motel chain back from when I used to do a LOT of business travel. The E-ink connected to the cloud via my wireless router so I could read new books anywhere in and around the motor home. Using my sister in law's address I got a library card from the Mesa county CO library system. Now I could download new books from them while in AZ for the winter.
Now I was hooked and bought my wife a Kindle PaperWhite reader.It has a back lit display. While I was at BestBuy doing that I got myself a KindleFire. Then I had to get my wife one also. So now we have 4 Kindle devices.
The neat thing about the cloud is that with a WiFi connection any device can be synced to the last page read in any book I open.
I use the E-ink as my reader in the library (bathroom) and my other two at places where I might have to wait a while like a doctors office. I can download books to the Kindle device so I don't need a WiFi connection to read them.
I use the Kindle Fire if I want to get on the internet when I am out in the world, know I will have a WiFi connection and may want have access to the internet, otherwise I take the PaperWhite. It's lighter and has a longer battery life.
My wife exclusively uses her Kindle Fire for game playing and web surfing with the browser.
I have yet to buy an ebook. Amazon offers free ones everyday and a bunch of web sites do too. Many are in the epub format rather that the Kindle mobi format but I use the Calibre app mentioned above to convert them.