Expressions that are distinctly parenthetical should be set off with commas. Providing supplemental information, they interrupt the flow of a sentence or appear at the end as afterthoughts.
Commas should set off nouns of direct address, the words yes and no, interrogative tags, and mild interjections.
Note that these examples are more characteristic of speaking than writing.
Exercises:
Major uses of the comma 1
Major uses of the comma 2
All uses of the comma