An apostrophe is used to show possession and to indicate where a letter has been removed from a word. It is placed after the possessive noun to indicate something belongs to that noun, such as "Amy's ballet class" or "the Jones' lives." An apostrophe is also used after plural nouns not ending in 's', such as "the parents' bedroom." Other uses include contractions where two words are combined, like "they've" from "they have." The document provides examples of correcting use of apostrophes in a passage about a family and their busy week.