"SmartSets are sets on Flickr that are updated for you in an automated way. For example, if you have a SmartSet based on the tag "flower", anytime you add photos to flickr tagged with "flower" they will automatically be added to your flower set for you (after you click refresh on SmartSetr)."
This is pretty nice. Kind of works like Smart Folders in Mac OS X.
By way of LifeHacker...