CMYK — 4 Color Process — Acronym for the four process colors, Cyan, Magenta, Yellow and Key (black). (This is used for CMYK print applications.)
PANTONE — Based on a color swatch-book system that gives printers recipes for how to mix inks to match the colors shown. (This is used for print applications.)
RGB — Acronym for the three colors that make up monitor images, Red, Green, and Blue. (This is used for on-screen applications.)