Both these verbs mean "to be" in Spanish, whereas in English we only have one. This can make it confusing, but there is a general rule you can use to remember which you have to use. "Ser" is used for things which are permanent and usually unchangeable. For example: nationality (soy inglesa), physical appearences (es alto), characteristics (eres inteligente). "Estar" is used to describe a state, something which can change, and also to describe something. For example: temporary feelings (estoy cansada), location (está en Espana), description (estaba oscuro).