How a silhouette is created?

The camera lens and human eye see a silhouette when there is a bright light in the background of a subject. Any object which is placed right in front of a brightly or strongly lit background it creates a silhouette effect. The object in the foreground looks black contrasting with the strong lighted backgrounds. However, if your subject is lit from the front, you may not see the silhouette effect.

Any strong bright back-light that's coming from the back of the subject / foreground creates the silhouette. Technically, the contrast between the bright background and the dark foreground object is so large the camera image sensor, cannot resolve details in it, leaving it black.

In photographic terms, "if there is more than two stops difference in the light between the foreground object and the background lighting there will be a silhouette". So, whenever the subject (foreground) is not lit from the front and there is a bright light in the background, like the sky, fire, light etc. which is relatively bright compared to the foreground a silhouettes will be created.

Talking about silhouettes, clarity is everything. You need to be doubly sure that the  edges of your silhouette are well defined and not confused with clouds and other objects as well as they are sharp. The shape and the focus are the two main important things. If your picture lacks sharpness & clarity, it looses the simplicity & purity as a silhouette.

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