Just below the Source list, iPhoto can display any of three useful panels:
Information. On this panel, you can view and edit general data about a photo, album, roll, or whatever else you've selected.
Calendar. This feature, new in iPhoto 5, is fantastically useful. It helps you pluck a photo out of your thousands according to the timeline of your life.
Keywords. This option was in previous versions of iPhoto, but its location is new. As before, the idea is that you can use this list of keywords for tagging your pictures with text labels, from "Robin" to "sunny day" to "prize winner."