To summarize and make it simple, let's talk about what iCloud.
ICloud is a free service offered by Apple and offers you to store photos, email ... on the apple servers but also to synchronize your data between your iPhone, iPod touch, iPad, your computer or with the site iCloud.com which we will talk a little lower.
iCloud also allows you to re-download ad vitam eternam applications already purchased on Appstore.
Before you start talking a little storage. iCloud is 5GB free for all. 
"5Gb is not much!", I hear you ... But your music purchases, apps and books as well as your Photo Stream are not included in this free space. Only your mail, documents, contacts and apps data will be saved. So it leaves you a little margin by regular mail sorting for example.
To finish with storage if you want to offer a little more, here's what it will cost

Back to your iPhone. To activate iCloud, go first in the settings menu, which should start smoking with all the settings you made ​​since iOS5. You will find a section dedicated iCloud with a pretty little cloud on which you will click. You are now faced with this screen.

Start with set your iCloud account with your Apple ID. If you have several (a US and UK for example, it is not a problem, you can either choose one of two (or even create a third), it may also be helpful to read this article about iTunes account sharing in a family to purchase apps, they advise to create another identifier for iTunes.

Once done 9 items await you:
  •  Mail: keep (or not) your email in the cloud
  •  Contacts: Your effortless address book on all your iDevices
  •  Calendar: more excuses for missed appointments. Tuck them into the iPad and they will find themselves directly in the iPhone or even the Mac. Convenient
  •  Reminders: Reminder is new iOS5, a kind of "todo list" which has also its place in the cloud
  •  Bookmarks: Start playing a note on iPhone and iPad finish is now possible by enabling this option.
  •  Notes: You have big fingers? Type your notes on the Mac and they will sync with the iPhone to take them anywhere
  • Stream  pictures: Immediately taken, immediately sent to the cloud. It is now easier to share photos or even save the last ... if you like
  • Documents and  data: you have iWork for iOS. Now your documents are transmitted directly between your machines via the famous cloud
  • Find My  iPhone: No need to explain ...

Finally click on Storage and you can manage your backup storage and purchase additional GO. At the bottom of the page is a backup switch. If enabled, your film, accounts, documents and settings will be stored in the cloud and restore your machine when clearing can be made ​​from the cloud as well. Please note however that activating this function, these elements will not be stored on your computer. You choose ! You are warned!
Web side, the site iCloud.com also lets you access your data but this from a computer.

Go on www.iCloud.com and enter your Apple ID and password. Once this is done you can access your mail, contacts, calendar and your documents. You can also find your iPhone.

Finally, before you get to choose what you store or not on Apple's servers, be aware that a Web page allows to know the health status of Apple's iCloud servers, it is located here.
You're ready, you know the basics of iCloud, it remains only to get familiar with the service.
For those who started from last week with the release of iOS 5, What do you think?Advice to share?
Find our other tutorials iOS5 here.