Conceptual art is any work that questions the meaning of art itself.
It may not be obviously recognisable as art. For instance, a piece of Conceptual Art by Felix Gonzales-Torres consisted of a pile of candies on the floor. It may only exist for a moment, and only survive afterwards in notes or photographs which document the work without actually being the work itself.
Confused? Even the Conceptualists couldn't hit on a coherent definition. People have always asked questions about art (such as "You paid how much for that?!") but art that asks its own questions may safely be considered Conceptual.
Use the term whenever you see a work you don't quite understand. Chances are, you'll be right.