Simulated-process color separations make use of halftone dots (picture a closeup of a printed comic) to layer and blend opaque inks (essentially liquid plastic), to create highly detailed or photorealistic images. 

At it's most basic, a separator breaks down an image to as few essential colors possible, determining exactly what color inks to use, in what print order, and at what amounts for each screen (layer), to recreate all the blends and details of the original image. Listing every variable taken into account during the process would be excessive.

View my other projects featuring some of the below designs with animations simulating the printing process of each screen building to the final image.