The punctuation marks that signal the end of a sentence are the period, the question mark and the exclamation mark.
You use the period, by far the most common of the end punctuation marks, to terminate a sentence that makes a statement. You may also use periods with imperative sentences that have no sense of urgency or excitement attached:
Without a doubt, Lady Emily was much happier after her divorce.
Turn right at the stop sign.
Bring me a cup of coffee and a cheese danish.
When you want to express a sense of urgency or very strong emotion, you may end your imperative sentences and statements with an exclamation mark:
Look out below!
Leave this house at once!
I hate him!
Exclamation marks are, however, rare in formal writing. Use them sparingly, if at all.
You should use the question mark at the end of a direct question:
Who's on first?
Where is my flowered cape?
Be careful not to use a question mark at the end of an indirect question. Indirect questions are simply statements, and therefore end with a period:
I wonder who was chosen as Harvest King in the county fair.
She asked if she could play pinball.
The teacher asked who was chewing gum.