(Source: Grammarly.com)
The Parentheses
Parentheses are used to separate explanatory or qualifying remarks in a sentence. However, content within parentheses is typically not necessary to fully understanding the sentence. For example, “The summer solstice (between June 20 and June 22) is the longest day of the year.” Using parentheses to interject the possible dates of the solstice, as they vary from year to year, adds to the complete understanding of this sentence. But those dates are not strictly necessary; the sentence was fine without them.
The Dash
Oh, the dash. The dash has become so popular in news writing that it springs up everywhere. The dash is formed by two strokes of the hyphen on your keyboard, and comes in two distinct varieties: the “em dash” and the “en dash.” The em dash is a longer mark than the en dash, about the width of the letter “m.” The en dash is often reserved as a replacement for the word “to.” A good example of the en dash is the expression of a span of years, like “the years 1950-1970.”
Read more: http://www.grammarly.com/blog/2014/duking-it-out-parentheses-vs-dash/
