What is the difference between “her” and “hers”?
My book is better than her (book).
My book is better than hers.
I would recommend a good dictionary as your first port of call for questions like this. E.g.:
oxforddictionaries.com/definition/english/her
oxforddictionaries.com/definition/english/hers
Both ‘her’ and ‘hers’ are possessives used to indicate as 'belonging to some ‘SHE’. HER is possessive adjective and HERS possessive pronoun of SHE.
Usually for a name or a noun we add an apostrophe and ‘s’ to get the possessive meaning (e.g. John’s, Mary’s etc). But for personal pronouns (I, We, You, She …) we have fixed possessives like MY/MINE, OUR/OURS, YOUR/YOURS, HER/HERS …
When a possessive adjective (her) is used, it needs to be followed by a noun. So the word ‘book’ is necessary (though repetitive) after ‘her’ in the sentence ‘My book is better than her book’. However, for a possessive pronoun (hers), the noun that follows is redundant, because it is supposed to be there already. It can be understood from the context. For instance, 'This is my book, that is yours. ‘That is yours’ means that ‘that is your book’. By using the possessive pronoun form (yours) we can avoid the repetition of ‘book’.