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.