Not trying to get off topic but is Kindle any different from other book apps available?
Kindle is
Amazon's electronic publication platform. If you buy a book from
Amazon, you can only read it on one of their Kindle devices, the Kindle for PC program or on an Apple mobile device through the Kindle app. The Kindle app is not a "universal" reader. It is only for stuff you get from
Amazon.
Likewise, the other two major "brick & mortar" bookstores (Barnes&Noble and Borders), have their own iPad apps that work in essentially the same way.
With the iBooks app, not only can you read the stuff you buy from Apple, but you can also use it to read "generic" ePubs in addition to PDF's. Unfortunately, the depth of their content in terms of availability stinks... at least compared to
Amazon.
The best (IMHO) "generic" reader is called GoodReader... and it really is! It isn't free like the other reading apps. It costs a whopping 99¢! There are links within the app to content providers, both paid and free, but it will also read just about any public domain format out there. Another advantage to GoodReader is that you can store your stuff on a "cloud" like DropBox. And yet another advantage is that you can transfer files directly to your iPad without syncing (but you do have to do it through the iTunes program on your PC) or the method I used most often is through your home WiFi network thereby bypassing iTunes altogether.
Clear as mud, eh?