Parentheses

Use parentheses ( ) to include material that you want to de-emphasize or that wouldn't normally fit into the flow of your text but you want to include nonetheless.

If the material within parentheses appears within a sentence, do not use a capital letter or period to punctuate that material, even if the material is itself a complete sentence. (A question mark or exclamation mark, however, might be appropriate and necessary.) If the material within your parentheses is written as a separate sentence (not included within another sentence), punctuate it as if it were a separate sentence.

Examples:
Forty-three years after his death, Robert Frost (we remember him at Kennedy’s inauguration) remains America’s favorite poet.
Forty-three years after his death, Robert Frost (do you remember him?) remains America’s favorite poet.
Forty-three years after his death, Robert Frost remains America’s favorite poet. (We remember him at Kennedy’s inauguration.)

Source: http://www.nationalpunctuationday.com/parentheses.html