I haven't used this new feature yet, but there is a nice introductory page here:
http://www.apple.com/icloud/features/photo-stream.html.
It's all about photos in the cloud though. So all of your iDevices must be iOS 5 and be set up to use the cloud. Then when you snap a pic, I assume, you have the option to store that pic in the cloud....which you can then access from any idevice you own.
That's what I'm getting from a quick glance.