"She" is also used after "being verbs" such as, "is", "are", "was", "were", "be", "been", "has been", ect. Ex: "I am she" and "We can hardly believe it was she all along! ".
"she" is the sentence subject whereas "her" is the object. If the one refered to is doing the action, use "she," if the action is done to her, use "her."
"She" is used as a pronoun, which means it replaces a noun. For example, you would say "She likes cake." "Her" is used as an adjective, to show possession. Thus, you would say "Karen likes her cake" to show that it is Karen's cake.
