Halftone is a printed picture that uses dots to simulate the tones between light and dark. Because a printing press cannot change the tone of ink, it will only print the ink color being used on press. This works well for printing text or line art: the press simply puts a full dose of ink for each letter or line on the paper, creating small solid areas of ink.
But black-and-white photographs are continuous tone images, and printing a photograph this way would have the same result: large solid areas of ink. White areas of the photograph would have no ink; black areas would have black ink; and gray areas would have black, not gray ink.
The halftone mimics the continuous tone of a black-and-white photograph by converting the picture to dots. Photographing a continuous tone image through a screen creates a duplicate image made of dots. Darker areas of the photograph have bigger dots and lighter area of the photograph have smaller dots. To the human eye, the black of the dots blend with the white of the paper to create shades of gray. The result is strikingly similar to the continuous tone of a photograph. See also continuous tone, duotone, four-color process, quadratone, screen, tritone.