The possessive case of a noun or pronoun shows ownership or association.
Nearly all nouns and indefinite pronouns show possession by ending with the s sound. This is spelled with and apostrophe plus an s.
For the rules on this see the "Apostrophes" heading in the Punctuation Contents.
Examples: Francine's sweater
(The sweater owned by Francine.)
(The grandfather associated with George.)
Possessives normally modify nouns as in the examples above, but sometimes they stand by themselves as a noun rather than as a modifier.
Example: That sweater is Francine's.
The possessive case of personal pronouns is irregular. Some possessive pronouns like my and your modify nouns. Others, like mine and yours stand alone and do not act as modifiers.
For more on this, see Possessive Pronouns.