Previously, you could easily import your iPhoto Library into Aperture but there was no going back from Aperture to iPhoto. Now with the unified iPhoto/Aperture Library, you can access and work with all of your photos in either app.
In addition to the unified Library, both apps have been updated for the new MacBook Pro with Retina Display.
The update to the latest version of iPhoto is available through Software Update.
Aperture costs $80 and is available in the Mac App Store.