I agree partly with DGaleano, who said that "Estar es uno de los verbos mas complicados en todos los idiomas."
In English, "estar" (to be) it is pretty straightforward, but English speakers (my American students are a good example) have problems distinguishing "ser" and "estar."
Here are the differences, explained with two acronyms:
SER (TO BE)
Ser is used to classify and identify permanent or lasting attributes. This acronym will help you remember the rule: DOCTOR (Description, Occupation, Characteristic, Time, Origin, and Relationship).
1) Description. Describe him or her
YO SOY ALTA
YO SOY HOMBRE
2) Occupation or profession
YO SOY ESTUDIANTE
ELLA ES ARTISTA.
The indefinite articles un, una, unos and unas are omitted when describing an occupation after the verb ser
3) Characteristic. Define the essential qualities of a person.
ELLA ES INTELIGENTE
4) Origin. Describes the place a person is from or the material something is made from
SOY DE NUEVA JERSEY
LA MESA ES DE MADERA
5) Time. Includes days, dates, and hours. For hours, use es for 1 o´clock and son for all other hours
ES LA 1 PM Y HOY ES MIÉRCOLES
LUISA ES MI MADRE, MARCOS ES MI AMIGO
Even after someone dies or someone breaks up, relationships are described using ser. Religion are also described using ser since a religion is considered a relationship with a higher power. Relationships are a form of identity.
ESTAR (TO BE)
Estar is used mainly to indicate temporary situations or conditions and locations. This acronym will help you remember the rule: PLACE (Position, Location, Action, Condition, and Emotion).
1) Position. Describes the place or posture a person or thing is in.
EL ESTÁ SENTADO
2) Location. Describes where someone or something is permanently, temporarily, actually, or conceptually
YO ESTOY EN LA CIUDAD, LA CLASE ESTÁ ALLÁ
3) Action. Describes an ongoing action and uses the present progressive tense. In Spanish, death is an ongoing action, not a permanent state
NOSOTROS ESTAMOS ESTUDIANDO, MI ABUELO ESTÁ MUERTO
4) Condition. The state of an object
EL COMPUTADOR ESTÁ DAÑADO
5) Emotion or feeling. How a person is feeling at a certain moment, physically or emotionally
YO ESTOY FELIZ, ELLAS ESTÁN CANSADAS, ÉL ESTÁ SENTADO