A smart filter is portable (you can carry it about), shareable (you can give it to your friends) and interoperable (it works on different application and websites from different vendors). A smart filter is a set of complex nested conditions/criteria/statements. A smart filter can be static (results and cached and remain the same) or dynamic (constantly changing) or combinations of the two; explicit (drag and drop your items into a container) or algorithmic (based on a formula) or combinations of the two.
A smart filter can be used to search for things; can describe the way results should be displayed; can describe an access control list (who can see/do what); can facilitate in contract/trade negotiations (offers and requests can be matched based on their criteria – the distance between offers and requests can also be determined when there is no definite match, with hints given to both parties as to how they can “come a little closer” to complete a trade transaction).