Simply put, ser is used for permanent states or conditions and estar is used for temporary feelings or states, but it is a little bit more complicated than that:
- Estar is used for locations, regardless of permanence. So one would use estar even for saying that the United States is in North America, which will not change. Estar is also used to describe feelings, even death (don't think about that too much).
- Ser is always used when describing the time of day, despite the fact the time of day changes constantly. Ser is also used along with characteristics (él es grande), nationalities (soy de México), professions (somos maestros), and religions.
Also, both verbs are irregular, so their conjugations have to be memorized:
|You are (Tú)||Eres||Estás|
|You are (Usted)||Es||Está|
|You are (Vosotros)||Sois||Estáis|
|You are (Ustedes)||Son||Están|
Duolingo Lesson: www.duolingo.com/skill/es/To-Be%3A-Ser_Estar