Now about the iCloud , what would I need it for since I use kindle
It's kind of tough to explain iCloud succinctly, since it does various things and is still evolving.
Primarily, iCloud is a service provided by Apple for free that allows you to sync certain important info automatically between Macs and iOS devices. This includes contacts, notes, reminders, calendar info, bookmarks, music, photos, videos and yes, PDF and iBookstore e-books.
Apple has provided a set of web apps (which you use through a web browser, say on a Mac or PC) that mirror the apps on your Mac or iOS device that sync (ie Mail, Contacts, Reminders etc) so that you can manage them from virtually anywhere, even someone else's computer.
There is a little bit of free storage space (about 5GB) for holding things, but this part of iCloud is at present underdeveloped except for iWork (Pages/Numbers/Keynote) users. Apple also gives you an @icloud.com email address that is ad-free and likewise syncs across devices.
I love having notes and reminders instantly synced on all my devices, I like seeing photos I just took with my iPhone show up on my Mac for editing, and I like being able to update contacts or calendar info and not have to worry about the change not appearing on my other devices. Some third-party apps use iCloud for syncing as well, and again its very handy and automatic.
Kindle takes care of its own syncing across devices -- all you have to do is have the Kindle app installed on any device you want to read your Kindle ebooks and you're all set. If all you're wanting to synch is e-books, then you can stick with Kindle/Amazon and not worry about iCloud, but if you want to buy books from Apple's iBookstore and/or enjoy iCloud's other benefits, you might consider signing up.