Full Idea
The extensional presentation of a concept is just a list of the objects falling under the concept. In contrast, an intensional presentation of a concept gives a characterization of the concept, which allows us to pick out which objects fall under it.
Gist of Idea
Concepts can be presented extensionally (as objects) or intensionally (as a characterization)
Source
Michèle Friend (Introducing the Philosophy of Mathematics [2007], 3.4)
Book Reference
Friend,Michèle: 'Introducing the Philosophy of Mathematics' [Acumen 2007], p.64
A Reaction
Logicians seem to favour the extensional view, because (in the standard view) sets are defined simply by their members, so concepts can be explained using sets. I take this to be a mistake. The intensional view seems obviously prior.