An interjection is a word or group of words that expresses emotion.
Hey, have you heard the news?
(The interjection "hey" expresses excitement.)
Use an exclamation point to show strong emotion, and treat the interjection like a separate sentence.
The Lady Falcons Volleyball Team is headed to state. Terrific!
Use a comma to show mild emotion, and treat the interjection as part of the sentence.
Indeed, they've fought their way to the top with a 14 and 3 season.