In English, there are three main uses for apostrophes, one of which (number three below) is going out of style.
1. Forming possessives
Apostrophes create possessive nouns. Singular nouns are usually made possessive by adding an apostrophe followed by an s (e.g., the book belonging to the boy becomes the boy’s book). Plural nouns are usually made possessive by adding an apostrophe (e.g., the houses belonging to the families becomes the families’ houses). There are exceptions. For instance, irregular plural nouns that do not end in s (e.g., women) are usually pluralized in the manner of singular nouns (women’s).
2. Standing for missing letters
An apostrophe takes the place of an omitted element in a word. For example, it stands for the missing letters no in the contraction can’t (short for cannot).
Apostrophes are sometimes used to pluralize acronyms, numbers, or letters—for example, 1970′s, ABC’s, FAQ’s. But this type of apostrophe is becoming less common, and many English authorities now recommend against it. Many 21st-century publications instead use 1970s, ABCs, FAQs.