"With CMYK printing, halftoning (also called screening) allows for less than full saturation of the primary colors; tiny dots of each primary color are printed in a pattern small enough that human beings perceive a single color. Magenta printed with a 20% halftone, for example, produces a pink color, because the eye perceives the tiny magenta dots and the white paper between the dots as lighter and less saturated than the color of pure magenta ink.
Without halftoning, the three primary process colors could be printed only as solid blocks of color, and therefore could produce only six colors: the three primaries themselves, plus three complementary colors produced by layering two of the primaries—cyan and yellow produce green; cyan and magenta produce a purplish blue; yellow and magenta produce red (these subtractive complementary colors correspond roughly to the additive primary colors). With halftoning, a full continuous range of colors can be produced."
Wikipedia
Screen angle
"To improve print quality and reduce moiré patterns, the screens for individual colors are set at unique angles. While the specific angles depend on how many colors are used and the preference of the press operator, typical CMYK process printing uses any of the following screen angles:
C 75° 15° 105°
M 15° 45° 75°
Y 0° 0° 90°
K 45° 75° 15°"
Wikipedia
Through a little bit more research I found that the term "moire patterns" or "moire effect" means that the dot pattern used to create an image is more noticeable... the viewer doesn't see a cohesive image, rather a blurred image or an image where the dot pattern is highly visible as the dots are spaced too far apart. This can be improved by changing the halftone settings and the angle of the screen.
