Don't confuse it with a shadow. A silhouette is a dark outline seen when looking towards light, where a shadow is a dark shape projected onto a surface with the light behind it.
Some tricks on taking good silhouette shots:
1. Make sure your light source is facing you, and the object is between you and the light (e.g. a sunset.)
2. Choose your object carefully - only the edges will be seen, so it must be interesting enough to keep the viewer. E.g. cardboard box - probably bad, bicycle wheel - probably good. When photographing people in silhouette, try to capture faces in profile.
3. Make sure the light space has its own interest, as the silhouette will probably be black. If there is no texture or colour in the background it may look like a vector image, and not a photograph.
3. Keep the frame uncluttered. Too many silhouettes will confuse the image.
4. Light settings should optomise the lit part of the image, not the object. Never use flash.
5. Focus should be on the object (the silhouette), not on the light source.
In saying that, the picture above breaks one of the rules (3) completely, and one partially (2 - no faces in profile), however saying that, the silhouette is not completely dark, and I'm a bit of a rule-breaker anyway...
Submitted for Photohunt on tnchick.com