Halftones, magic things they are. Only people in the know can do them.
Any postscript printer can generate them. Corel can convert an image to halftones.
Used for offset and screen printing mostly. DYK, when used in a four color process each color must me aligned on a certain angle or the halftones will cause a morray. Like on TV when some pattens on a shirt will cause a funny effect, or if you look at something through a screen and move. More useless knowledge from the "MOOSH!"