As with Facebook, there may be photos in your iPhoto library that you don't want the whole world to see. Now, you could delete those photos or remove them from iPhoto, but an easier and more temporary solution would be to simply hide them. It's a snap to hide photos in iPhoto '11. Hidden photos remain in your iPhoto library--they are just removed from view for those times when you are showing photos to a friend or family member who doesn't necessarily need to see the full breadth of your photographed life. You can hide individual photos, or you can hide an entire iPhoto Event. You can also hide photos based on iPhoto's Faces and Places features.
To hide a photo, mouse over its thumbnail and you'll see a small down arrow in the lower-right corner. Click on the arrow and you'll see the Hide option below the orange X. You can also find the Hide Photo command in the top menu under Photos, or you can use the keyboard shortcut Command-L.
To unhide a photo, you can use the same keyboard shortcut or choose Unhide Photo from the Photos menu option. Or by mousing over a thumbnail of a hidden photo, you can use the small arrow in the lower-right corner to select the option that is, oddly, labeled Show instead of Unhide.
If your current view is set up to view hidden photos, you'll see an orange X in the upper-right corner of a hidden photo and its thumbnail. To remove hidden photos from view, choose View Hidden Photos to remove the check mark next to that menu item.
To hide an entire Event, open an Event and highlight all of the photos in it. Then use one of the three options outlined above to hide the photos.
Hiding photos using iPhotos Faces or Places feature works the same way. For Faces, you select a person from your Faces list to open the photos of that person. Then you can choose which photos to hide. For Places, select a Place from the map to open up the photos of that place to select which photos to hide. Suddenly, last semester's paramour or that week in Cancun never happened.