Ser vs. Estar
Two verbs that both mean 'to be'
Both ser and estar translate as 'to be', but they cover different territory. Ser describes essence: who or what something is. Estar describes state: where, how, or in what condition something is.
Use ser for
Identity, origin, profession, nationality, time, dates, possession, and inherent characteristics. 'Soy profesora', 'Es de Madrid', 'Son las tres'.
Use estar for
Location, temporary states, emotions, ongoing actions (with gerund), and the result of a change. 'Estoy cansada', 'Está en la cocina', 'Estamos hablando'.
Same adjective, different meaning
Some adjectives change meaning with ser vs. estar. 'Es aburrido' = he is boring; 'Está aburrido' = he is bored. 'Es listo' = he is clever; 'Está listo' = he is ready.
Ejemplos
"Mi hermana es médica."
My sister is a doctor.
"Mi hermana está en el hospital."
My sister is at the hospital.
"La sopa es de tomate; está caliente."
The soup is tomato; it's hot.