The Faces feature in iPhoto uses face recognition to help you find and name the photos of your favorite people, which then appear on your Faces corkboard.
When you open iPhoto ’11, iPhoto automatically scans your photo library and groups likely matches. After the scan is done, click Faces in the Source list so you can get started with telling iPhoto who is who (by naming people in the photos), and confirming or rejecting the matches iPhoto suggests.
When you import new photos, iPhoto scans them to see if any include people on your corkboard, and then suggests selected photos for your confirmation.
To get started with Faces:
Name a specific friend or family member, or have iPhoto suggest unnamed faces found in your library. For more details, see this topic: Name the people in your photos.
Click to select Faces in the Source list to see your Faces corkboard, which now shows the people you’ve named. Double-click a person’s snapshot to see suggested matches you can confirm or reject.
If you have a photo in your library that shows a person whose face wasn’t identified by iPhoto, you can add and name the missing face.