A halftone is a reproduction of a photograph on paper. That's the definition, but if you want learn more about the technology behind halftones, read the next two paragraphs. To learn more about creating halftones with your scanner, go to the next page.
To reproduce the gray tones of a black-and-white photograph, printing services convert the photo into a series of black dots so small that the human eye blurs them together with the surrounding white paper. The lighter areas of the photo are created by small dots and the darker areas by large dots.
You're more limited when you try to reproduce a photo using your personal computer. Since your output device can only make dots of one size, it has to group the dots together into halftone cells. Then it reproduces the lighter tones by turning some of the dots off in each cell, while turning more of them on to create the darker tones.