Lenses in strobe and lighting situations refer to optical snoots or fresnel lenses that are mounted to the lighting unit. Fresnels use concentric rings of glass to focus light in a direction - which makes them great for projecting a sunlight effect with soft falloff with the shadows. 

Spotlight lenses or optical snoots use condenser lenses & iris (or gobos) to focus light with hard defined shadows. Most optical snoots can carry many different gobos, some even allow you to mount negatives for double exposure effects.