iPhoto '11: Events
iPhoto organizes the photos in your photo library by Event, which groups photos taken during a certain period of time, such as during a single day or during a two-hour period. (You can change this time period in iPhoto General preferences.)
Each Event group has a key photo, which appears as the representative photo in the main viewing window.
You can move photos from one Event to another, merge or split Events, or rename an Event, among other options.