Apple's release notes:
- Photos now can be added to shared streams or My Photo Stream by dragging them to "Photo Stream" in the source list
- Multiple email addresses can now be copied and pasted in the "Shared with" field for shared streams
- When more than five subscribers "Like" a photo in a shared stream, their names are now displayed correctly
- Contextual menu now includes an Import command to import all the photos contained in a shared photo stream
- Addresses the reliability of Shared Photo Streams when switching between iPhoto and Aperture with the same library
- Fixes an issue that could prevent Microsoft Outlook from being used to email photos from within iPhoto
- Updated national holidays are available for use in printed photo calendars
- Includes stability improvements
The update is available for download from the Apple website, or you can update it through the Mac App Store.
iPhoto is available from the Mac App Store for US$14.99 and is part of the iLife suite that is included with every new Mac. The update is free of charge.