This 45" White Satin Umbrella with Removable Black Cover from Westcott can be used in standard bounce mode with the black cover installed or in shoot-through mode like a softbox with the cover removed.
Umbrellas soften, broaden, and diminish the light output of any tungsten or flash light source. Umbrellas with a white interior will soften and weaken light more than an umbrella with a silver lining. A white umbrella without a black backing may also be used as a makeshift softbox, although the use of the light will not be as efficient as with a softbox.
Choosing umbrella size is determined by the size of the subject and the strength of the light supply being used.