Umbrellas are one of the most simple and easy to use light modifiers available. They are considered a broad light source and soften the light in varying degrees based on the size and shape.
Umbrellas usually come in 3 different types:
Reflective: The interior of the umbrellas is either Silver, Gold or White reflective material and increases the size of the light source by reflecting it back over a wide area.
Diffused: White Diffuser Umbrellas work on the same principal as a softbox but are much less direct than a softbox. The light is diffused thru a white material by shooting the light thru the material on to the subject.
Dual Umbrellas: A combination of both Diffused and Reflective Umbrellas. The best of both worlds! Dual Umbrellas have a removable backing (usually satin white or silver) that allows the umbrella to be used as either a reflective or diffused umbrella.