Pronouns are different in nature: she is a nominative pronoun; Her is an object pronoun and an adjective that modifies nouns.

Different meanings: she is the subject, which means "she". She means "she" as an object; Her is an adjective possessive pronoun, which means "she".

Different positions: as the subject, she usually appears at the beginning of a sentence. She is used as an object, usually in a sentence or at the end of a sentence, and also as an adjective possessive pronoun of the object, usually before a noun.