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A halftone is an image or printing technique that "simulates continuous tone through the use of dots varying either in size, in shape or in spacing." In the Illustrator and Photoshop worlds, there's long lists of halftone filters, swatches, and compositing techniques used for printing and newsprint, comic, retro, and other effects. But the fun of After Effects is animation, and some techniques may not be immediately apparent without built-in filters.
If you've never used the Color Halftone effect in Photoshop, check out how a gradient is effected by the filter in Creating Synthetic Backgrounds in Photoshop: