Short answers, pulled from the story.
Silhouette animation is a form of animation in which characters appear only as black silhouettes, typically created by backlighting articulated cardboard cutouts. It is inspired by shadow play and by silhouette cutting but remains technically distinct from shadow play itself.
The earliest known silhouette animation is The Sporting Mice, a 1909 short by British filmmaker Charles Armstrong. German animator Lotte Reiniger, who likely did not know of Armstrong's work, established the format's standard practices with her 1919 film Das Ornament des verliebten Herzens.
Die Geschichte des Prinzen Achmed was released in 1926 and ranks among the oldest animated feature films. Its release sparked a revival of interest in silhouette animation and inspired imitators as far away as Japan.
Traditional silhouette animation is made by cutting figures from paperboard, sometimes reinforced with thin metal sheets, and tying the joints together with thread, wire, or paper fasteners. The figures are moved frame by frame on an animation stand and filmed from above with a rostrum camera.
Modern silhouette animation appears in South Park during scenes when the lights turn off, in a 1999 episode of Mona the Vampire, and intermittently in Sayonara Zetsubō-Sensei in 2007. The Australian short Jasper Morello, from 2005, uses a style close to Prince Achmed and was later nominated for an Academy Award.
Silhouette films use varying shades of grey in the background because the paler the shade, the more distant an element is meant to appear, creating an illusion of depth. The foreground remains solid black, following the format's traditional monochrome convention.