iPhoto Photo Stream is the iCloud service that uploads and stores the last 30 days of your photos and automatically pushes them to all your iOS devices and computers. It lets you view all your recent photos on all your devices without having to sync—all you have to do is turn it on.
How Photo Stream works
First, turn on Photo Stream on any devices or computers you want to include.
Next, take a picture with your iOS device, or import some photos from a digital camera into your iPhoto library.
Photo Stream automatically uploads the photos to iCloud, and then downloads them to iPhoto and your other devices (via Wi-Fi or Ethernet).
The downloaded photos appear in Photo Stream view in iPhoto and in the Photos app on your iPad, iPhone, or iPod touch.
By default, every photo you take or import using any device or computer in your Photo Stream is uploaded automatically. You can modify this setting in iPhoto.
iCloud stores your photos for 30 days, to give all your devices enough time to connect and download the photos. Photo Stream keeps a rolling collection of your 1000 most recent photos on your devices. To keep photos on your device permanently, you can save them to your device’s Camera Roll or another album.
By default, iPhoto automatically imports all your Photo Stream photos into your iPhoto library. You can change this setting in iPhoto.
To use Photo Stream, you need iPhoto 9.2 (or later) and Mac OS X Lion v10.7.2 (or later) on your Mac, iOS 5.0 (or later) on your devices, and an iCloud account.
For more information about using Photo Stream, click a help topic on the left.