The silhouette is a black shape that stands out. it has a uncomplicated background. Making a silhouette can be easy as well as complicated, the creation of such a photograph is not always easily accomplished. The person taking the picture must learn to deviate from the rules generally followed when taking a normally- lit picture by adjusting for a different relationship between light and subject.
To make a silhouette you need good lighting with not much of a background. The situation that best lends itself to the silhouette is when there is a subject with a large and relatively simple shape in the foreground which is strongly lit from behind. This is a lighting situation that is best avoided when shooting normal exposures, but it is ideal for creating a silhouette. In order to achieve an exposure that will result in a black, distinct form against a bright, crisp background.