Poly-chokes vs. fixed chokes vs. screw in chokes. A quick explanation.

I recently had a question come through about the savage super choke on the Stevens 320B. Thought i'd take the time to do a short explanation of the differences between/intended purpose of the 3 major kinds of chokes you're likely to encounter on shotguns.

Firearms in order of appearance:
Stevens 320B
Ithaca Model 37
JC higgins model 20
Beretta A300