An apostrophe is a punctuation mark used in contractions to replace missing letters. The contraction "we'll" stands for "we will," with the apostrophe replacing "wi." It can also show possession, as in "Mary's car." The apostrophe indicates the car belongs to Mary.
To correctly pronounce apostrophe, accent the second syllable: "uh-POSS-truh-fee." In addition to being a punctuation mark, apostrophe can also be a literary device in which the speaker of a poem talks to someone who is not there. A famous example of this is Walt Whitman's "O Captain! My Captain!" in which he addresses the deceased Abraham Lincoln: "O Captain! my Captain! rise up and hear the bells."