What is Steepener, flattener, curve fly?

A steepener is a position that gains when the yield curve steepens, typically long a shorter bond and short a longer one with equal DV01; a flattener is the opposite. A curve fly is a position in three maturities, long or short the middle against the two wings, sized to be neutral to the level and slope of the curve; its price is measured by twice the middle yield minus the two wings.