An em dash takes up the same amount of space in the text as a capital "M" in the chosen font. The width of an em dash is twice the width of an en dash and is also longer than a hyphen. Use one space on each side of an em dash. If you cannot type an em dash, use two hyphens instead.
Use an em dash for setting apart parenthetical phrases that provide a significant break from the rest of the sentence. Em dashes provide more emphasis than commas, colons or semicolons, but do not overuse an em dash when you only need a comma. You usually find em dashes in informal writing.
One example of an em dash where it takes the place of a colon is "I need to buy two things — milk and bread." Another example of an em dash is "I clean the house — you run the errands." An example of the em dash setting off a parenthetical phrase is "Please bring the dog — even if it misbehaves — back into the house."