The second important feature of the fresnel (although it is not unique to fresnels) is the sliding lamp. (Note: In production, the thing that gives off light is a "lamp," not a "light bulb." The device that houses the lamp is called the "instrument.")

Lamps inside a fresnel can slide forward or back. When the lamp slides forward, the beam becomes more diffuse and the light softens.

Similarly, sliding the lamp to the back makes the light harder and more directional. That's why fresnels are so popular--because they can provide hard light or soft light or anything in between.