The word, Silhouette, sounds provocative and mysterious to me and that's why I have explored this method of photography.  The definition according to:  http://www.amazon.com/Exposure-Digital-Field-Guide-Alan/dp/0470534907

The silhouette is a style of photography that purposely underexposes your subject so it is nearly black or is full black against a brightly lit background.  The basic idea is to place a recognizable shape against a bright background and expose the image for the background, which turns the subject black.

Picking your subject is most important as it will be dark without color so the shape needs to tell the story.  Let's check out some of the silhouette shots from my library…..

I took this from below stage level at my great-niece's recital.  She isn't totally dark but it does have a mysterious feel and falls into the silhouette genre, I think.  Loved shooting all the young dancers with the back-lit stage….good drama!  I had to pump up my ISO to 1600 and using my go to lens, the 24mm-70mm, I had a 1/100 shutter speed and 2.8 f-stop.  This next one is a bit racy…..

The shape tells the story yet leaves much to the imagination!  The background has to be lighter and brighter than the subject.  Let's take another one taken at a park during twilight…...

Just enough back light to silhouette this duck and her ducklings.  So, obviously, we're breaking the "don't shoot into the light" rule when we create these shots.  It's easy to find these opportunities.  The shapes of the fishermen in the shot below stood out from the beautiful fountain and well lit background…..

Their gestures and body language tell us they are trying to figure out where those elusive fish are hiding!  The trees are also subjects in this shot.  Let's look at one more …...

I think this one falls into the silhouette category because of the immediate reflective light around the geese and ducks.  What do you think?