A halftone image is made up of a series of dots rather than a continuous tone. These dots can be different sizes, different colors, and sometimes even different shapes. Larger dots are used to represent darker, more dense areas of the image, while smaller dots are used for lighter areas.
Halftone images use used in newspapers and magazines because it is a much more efficient way to print images. Since a halftone image is made up of discreet dots, it requires significantly less ink to print than a continuous tone image. As long as the resolution of the image (measured in LPI is high enough, the dots appear as a continuous image to the human eye. However, if you closely examine the images printed in a newspaper, you should be able to see the dots that make up the halftone image.