work in progress
A slicer is a program that converts a 3D model (STL in most cases) into instructions that the printer can interpret. The printer uses these instructions known as G-code to know what to do. The G-code tells the printer the path to follow (width, length, height/X,Y,Z coordinates), the speed at which to print and travel, the temperature of the bed/platform and nozzle/hotend, and the flow rate of the filament. Not all 3D printers use G-code as the instruction language but it seems to be the preferred standard probably because it is human readable unlike some other languages used with other 3D printers.
This is a list of some of the slicer programs available.