The quick answer is the reflecting umbrellas, shoot through umbrellas, soft boxes, and all the other light modifiers out there all provide slightly different light and therefore a different look to the subject.
Strobist A wealth of info on this site. See the topic selectors on the right side part way down.
Joe McNally An outstanding pro who uses strobes most of the time and studio lights some of the time. His books and videos are invaluable and inspirational.
Virtual Studio A virtual lighting setup. You place the lights and see the effect.
Ad this is just the tip of the iceberg
Jon Kandel A New York City Nikonian and Team Member Please visit my website and critique the images!