A gobo (short for "go between") is a device placed between a studio light and the subject for the purpose of casting a shadow. These can be as simple as a piece of cardboard, wood or metal. They have been used on Hollywood movie sets for years.
Some time ago I purchased some digital gobos from LayerCake. These are met to be added to a scene during post processing. Here is an image of Tiffany where I used two such gobos:
Gobos work especially well with black and white images, and help to give a vintage look. In this instance, the use of hanging ivy and window gobos suggests a wealthy millionairess sitting in her Beverly Hills mansion in 1930. They can be blurred, faded, darkened, lightened, stretched, compressed, flipped and rotated, giving a great many looks due to their versatility.
Moreover, photographers can collect their own set of gobos. For instance, one could take a photograph of a window, mask out the glass and window frame, desaturate the image, add a blur, and there you have a gobo that nobody else has.