An apostrophe ('), like a hook, indicates possession. — To show that something belongs to one person, the apostrophe goes before the s (e.g. woman’s hair); to more than one person, after the s (ladies' room). Apostrophes are also used to indicate a contraction (wouldn't, replacing the missing o). Just as important, constructions like 1920s (in reference to the decade) and high 80s (temperature in degrees) don't need the apostrophe.

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