What all of them have in common is that they're really huge, broad light sources that wrap light around your subject yielding clean highlights and rich open shadows. Due to the special shape of the umbrellas there's a unique mix of soft enveloping light with the kind of crisp detail you only get from reflectors.
7' Parabolic Umbrella (White/Black)
The fixed black backing on the white/black version ensures that all of the light output is on your subject instead of through the umbrella. You can also vary the light-to-umbrella distance along the shaft for a harder or softer look or for longer throw capability.
7' Parabolic Umbrella (Silver)
The silver interior version provides a more specular and contrasty look than the white umbrella. Again, you can also vary the light-to-umbrella distance along the shaft for a harder or softer light. This silver version is ideal for throwing the light over a long distance. Two of them, for instance could light an entire auditorium or a large graduating class.
7' Parabolic Umbrella (White Diffusion)
The white translucent version of this umbrella yields the softest light. Used in standard bounce mode, some of the light escapes through the material, slightly raising the ambient light level and lowering contrast while most of it is directed towards your subject. If you want a harder light, turn the umbrella around and aim the convex side at your subject and punch your light source through it.