In the area of photography, a silhouette is defined as an outline that appears dark against a light background. More specifically, it is where your subject appears as a plain black shape against a brighter background. It is an artistic photography expression that many photographers like to refine and perfect in their images. This effect can be achieved with any bright light source with the sun being the most common. In a sunset silhouette photo, the sunlight in the background is exposed correctly forcing everything else in the photo to be underexposed causing the effect.
When you are preparing to take a silhouette image, there are many things to keep in mind. These tips are equally effective for both digital and film photography. First of all, you need to make sure that there is not too much light on your subject, even if it is being reflected on to your subject the stray light will ruin the effect. If there is not enough light in the background, your subject will appear grey instead of black. The effect is just multiplied when you have multiple colors of bright lights in the background. Some photographers focus on artificial lights, others focus on the sun at certain times of the day, the possibilities are endless.
I usually take my silhouette images when the sun is just above the horizon. I prefer the time around sunset because the sun causes the sky to be brighter than everything else for greater contrast. Another technique I use is to align the sun directly behind the subject so it causes a glow effect around the main subject. I usually use a relatively big subject so it creates a more drastic effect then a small insignificant subject.
I always use a narrow aperture (high f/stop) so the camera captures the whole scene with a high depth of field so everything is in focus. I usually use the aperture manual mode on my camera so I can control what the aperture will be and then the camera automatically selects the right shutter speed necessary for the photo. If you are trying to create the effect with a point-and-shoot camera make sure you compose the photo with the background light by pointing the camera at the background. If you compose the image by pointing the camera at your dark subject, then the background will be over-exposed and you will not end up with a silhouette.