Where ser describes what something is, estar describes how it is at the moment — and where it is. Use it for:

• feelings and moods — Estoy cansada. (I'm tired.)
• temporary conditions — El café está caliente. (The coffee is hot.)
• location — Marina está en Madrid. (Marina is in Madrid.)

Pronoun	Estar	English
yo	estoy	I am
tú	estás	you are
él / ella / usted	está	he, she is / you (formal) are
nosotros / nosotras	estamos	we are
vosotros / vosotras	estáis	you (a group) are
ellos / ellas / ustedes	están	they are / you (formal, a group) are

Mind the accents: está means is, while esta without an accent means this.