The Amazon Cloud is extra space that you can use to store your books, apps or movies that exists on the Internet i.e. outside of your device. You can gain access to the cloud either via 3G or using a Wi-Fi connection depending on the type of Kindle you have.
The Amazon Cloud is provided free and entitles you to 5 GB of storage.
The Cloud is organized just like a normal disk drive so you have folders containing your various types of files:
The 5 GB free storage plan is available to all Amazon.com customers, worldwide so you can access it whereever you are providing you have access to the Internet.