The verb "ser" is used to indicate an attribute or an inherent quality that is not likely to change. The verb "estar" is used to indicate a condition which is likely to change.

Este río es muy ancho. - This river is very wide.
Hoy el agua del río está muy caliente. - Today the river's water is very warm.

Ser is often used to express the norm. Estar expresses a departure from the norm.

José es una persona tranquila pero hoy está muy molesto. - Joe is a calm person but today he is very upset.

Ser is used to signify equivalence or identity between two elements

Alberto es doctor. (Alberto = doctor) - Alberto is a doctor.

María es una buena persona. (María = buena persona) - Maria is a good person.

Ramón es colombiano. (Ramón = colombiano) - Ramon is Colombian.

Ser is used to indicate origin

Diego es de Perú. - Diego is from Peru.
Este tapete es de México. - This rug is from Mexico.

Ser is used to express possession

Esa bicicleta es de Gustavo. - That bike is Gustavo's.
Las llaves son de Julio. - Those keys are Julio's.

Ser is used to talk about time

Son las dos. - It's two.
Es la una y media. - It's one thirty.

Ser is used to talk about dates

Hoy es el cuarto de junio. - Today is the fourth of June.
Es jueves. - It's Thursday.

Estar is used to signify the location of an object

La escuela esta al lado del correo. - The school is next to the post office.

Estar is used to express the present progressive

Estoy escribiendo. - I am writing.
Juan está escuchando. - Juan is listening.

Estar is used to indicate a resultant condition: estar + past participle

Estoy cansado. - I am tired.
Estamos despiertos. - We are awake.