Silhouettes make for cool photos. There is no getting around that. As clichéd as they may be, there is something that makes you stop and look at silhouettes. For me, I enjoy the anonymity a photo acquires when all of a subject's colour and detail is drained away, and all you're left with is shape and form. Silhouettes help you appreciate different elements of a subject that might have passed you by before. Add into that that most are taken behind a stunning backdrop and a beautiful sunset, and you're onto a winner in the hearts of your viewers.
Silhouettes are not complex pieces of art, and they are not technically difficult to achieve. You do have to be in the right place at the right time, so pick a somewhere (unless you have a studio suitable for low-key lighting) where your subject can be fully silhouetted against the sky, as any background elements will also be silhouetted and anything without detail and colour will mesh into a single element. Generally speaking, you'll need to take the photo from high, as in on a hillside or off a building, or from very low, with the photographer on the ground looking up at the subject. You'll also generally want to be doing silhouettes with lower light backgrounds, so at sunrise of sunset, where the chance of putting the light source (the sun), directly behind your subject which is where it needs to be to achieve the effect.
Then set your camera's manual exposure settings down. Most DSLRs will have a manual exposure control somewhere on the top right quadrant of the camera body, and most will range from -3 to 3. Obviously, you'll want to be well into the minuses. You may have to take a couple of preparatory shots to judge the stops of light you want to take away. I would also recommend a very fast shutter-speed. This is because sharp lines will be everything in a silhouette, and a fast shutter-speed (as you can learn by reading my "Understanding Exposure" post) will help define the lines, and you don't need the extra light from a slower shutter-speed. Aperture is less important for a silhouette, just make sure that your subject is in focus.
Then, snap away, and make it as cool as you can. This one below I took in Sierra Leone. I think jumping goes hand in hand with silhouettes. Let me know what you think!