What is Extrusion?

You can go some place like dictionary.com and look up the definition for extrusion.  From that you will see that extrusion is simply the act of pushing some material through some resistance.  Odds are that you have played with an extruder and not even realized it.  Play-Doh even has a selection of extruders.  Even one referred to as an extruder.

On this site, when the word "extruder" is used, it will tend to be a device that has a barrel, one or more screws that convey a food product, and some sort of die.