An interjection is a word or group of words that expresses emotion.
Whoa! Check out Ramon's skateboard tricks.
(The interjection "whoa" expresses excitement or amazement.)
Use an exclamation point to show strong emotion, and treat the interjection like a separate sentence.
That's his kick-flip. Cool!
Use a comma to show mild emotion, and treat the interjection as part of the sentence.
Wow, his ollie is super clean.