The iCloud Control Panel is required to use Photo Stream, Contacts and Calendars (Microsoft Outlook 2007 or later required).

iCloud stores your music, photos, documents, and more and wirelessly pushes them to all your devices.  iCloud is seamlessly integrated into your apps, so you can access your content on all your devices.

Give iCloud Control Panel a try to fully assess its capabilities!

NOTE:
To create an iCloud account you need an iPhone, iPad or iPod touch with iOS 5, or a Mac with OS X Lion v10.7.2.