Final answer:
Nonrestrictive elements such as parentheses and dashes provide additional but nonessential information in a sentence. Parentheses tend to downplay this content while dashes tend to emphasize it, and both can be removed without altering the sentence's meaning.
Step-by-step explanation:
The definitions you're looking for pertain to the use of parentheses and dashes within sentences to enclose additional information. Nonrestrictive or parenthetical elements are parts of a sentence that add more information about a noun but can be removed without altering the primary meaning of the sentence. This is in line with the definition you provided, wherein parentheses — the pair of signs () — are used to include extra information that is not essential to the main point of the sentence. However, when compared with dashes, parentheses tend to downplay the enclosed content.
Dashes, which are the signs –, indeed serve a similar function of enclosing additional information. However, dashes often emphasize this information to a greater extent than parentheses. A dash can also convey a pause, represent a contrast, or signal a change in direction within the sentence. As in the example you provided where the use of the dash sets off an appositive phrase that provides important explanatory information.
An example to illustrate these concepts could be: The renowned author Mark Twain—who was also known as Samuel Clemens—wrote the American classic 'Adventures of Huckleberry Finn.'