Q: What is the difference between a Checklist and an Action?
A: A Checklist is a list of things that have to be "checked off" as complete in order for the activity to be considered done. Actions are things that can be performed while the checklist is incomplete or complete...
Actions can also be on the activity without a checklist so that when you go to complete the activity it allows you to run some action like sending and email, changing tags or starting another workflow. This way you can choose which Action you want to accomplish without activating all of them.