An apostrophe should be used to show the omission of letters in a contraction. It is also used with possessive nouns, but not with possessive pronouns such as "yours," "ours" and "its."
Singular nouns are made possessive by adding an apostrophe plus an "s," even when the singular noun already ends in "s." For plural nouns that end in "s," an apostrophe should be placed after the "s" without an additional "s" following it. With irregular plural nouns that do not end in s, an apostrophe followed by an "s" should be used, as in "children's." When two or more nouns share possession, an apostrophe followed by an "s" should be added to the last noun. In general, an apostrophe should not be used to form plurals, although from time to time they are used with words not frequently pluralized, such as in the phrase "no if's, and's or but's."