Sets-- a grouping of photos that you can organize around a certain theme, such as Our Family Vacation . You can use them to highlight your favorite photos, or make an album that is only pictures of your dog, or of photos you've taken with a particular camera.
The Organizr is where both Collections and Sets are created. It also allows you to perform common tasks on large batches of photos, such as tagging, changing permissions, or editing timestamps. It’s really rather handy.