An exclamation point is like a traffic cop's whistle; it demands your attention. Use an exclamation point for emphasis, usually to show surprise or some strong emotion. Exclamation points take the place of other end punctuation.
You've performed excellently!
Stop throwing those pillows this minute!
Tip: Exclamation points should be used sparingly. Try to rely on strong verbs, and not punctuation, to convey emotion.