her是形容词性物主代词: 只作定语，也就是说，形容词性物主代词+名词

hers是名词性物主代词：相当于：形容词性物主代词+名词，所以，名词性物主代词作主语、宾语、表语。如：

This is his book. That is hers.

hers相当于her book