Estar = to be (temporary)
Ser = to be (permanent)
You use estar for things that are temporary, or can change quickly: feelings, emotions, and location.
You use ser for things that are permanent and never change:
a) race, gender, nationality or origin, what something is made up of (material), family relationships
or things that take a long time to change:
b) personality, looks, weight and height, religion, job (career)
You also use it for things that only change when they are supposed to: time and date