Tenebrism is a style of painting in which light is used to focus attention on a single figure or particular figures in a composition, with the rest of the painting being in shadow and the background very dark. The contrast between the areas of light and dark tone add a sense of drama to the composition, and spotlights what the artist wants the viewer's attention to focus on.
The term tenebrism is derived from the Latin word meaning "darkness" or "shadows", tenebrae. The paintings by Caravaggio would fall into a tenebrist style.
If tenebrism sounds like what you'd call chiaroscuro, don't worry as the difference is arguably something only academics worry about, though tenebrism can be said to be more extreme in its differences between light and dark. Whether tenebrism or chiaroscuro is used also seems to depend on which country an artist was painting in, with the former more commonly applied to Italian painters working in Caravaggio's style.
See Also: Comparing Rembrandt and Caravaggio at the Rijksmuseum, Amsterdam.
