Parentheses may surround words, phrases, and sentences (though should not surround multiple consecutive paragraphs). When enclosing a full sentence in parentheses, the final punctuation belongs inside the closing parenthesis; otherwise, punctuate outside the closing parenthesis, as in the previous sentence. If parentheses within a larger sentence contain a whole sentence, only punctuate before the closing parenthesis if using a question mark or an exclamation point: I left yesterday (did I turn off the stove?) and shall return tomorrow (it’s been a short, but tiring, trip).
Parentheses serve numerous specific functions, of which the most common are interjections, notes, asides, and explanations: His name was Jonathan (but wanted everyone to call him Jonny). Parentheses for these situations may be optional; rephrasing, or rewriting with em dashes or commas may suffice if the writer wishes to avoid using parentheses. Citations, per certain style guides, require parenthetical references (Garner 746). Parentheses can also clarify text through definitions and übersetzungen (translations).
Garner’s Modern English Usage warns against overusing parentheses, as “they tire the reader’s eye . . . and deaden the reader’s interest.” I have not read this advice elsewhere, though I use them sparingly, far preferring em dashes for interjections and asides. Although a purely stylistic notion, parentheses’ vertical nature impedes the horizontal flow of the eye—far more than the sleek dash. For those who do prefer parentheses, remember that the lazy reader, even if well-intentioned, sees parentheses as optional and may skip them entirely.