The verb ser (full conjugation) is used to indicate an attribute or an inherent quality that is not likely to change.

The verb estar (full conjugation) 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 molesto.

Joe is a calm person but today he is 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.

Ser is used to indicate an inherent characteristic or quality

Julio es joven. - Julio is young.

Sergio es guapo. - Sergio is good looking.

Ser is used to indicate origin

Diego es de Perú. - Diego is from Peru.
Este tapete es de México. - This rug is from Mexico.
Ramón es colombiano. - Ramon is Colombian.

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 cuatro de junio. - Today is the fourth of June.
Es jueves. - It's Thursday.

Ser is used to express the material that something is made of.

La chaqueta es de cuero. - The jacket is made of leather.
El suéter es de lana. - The sweater is made of wool.

Ser is used to express the passive voice in the construction ser + past participle + por.

El concursante es entrevistado por el reportero. - The contestant is interviewed by the reporter.
El alfiler es atraído por el magnetismo. - The pin is attracted by magnetism.

Ser is used to express the idea of "taking place" or happening.
La fiesta es el sábado. - The party is on Saturday.
El concierto es dentro de dos semanas. - The concert is in two weeks.

Ser is used in impersonal expressions.
Es necesario ser puntuales. - It's necessary to be punctual.
Es importante recordar los cumpleaños de los amigos. - It is important to remember friend's birthdays.

Estar is used to signify the location of an object

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

Estar is used to express a changeable state or condition

Ella está enojada hoy. - She is mad today.
Hoy estoy enfermo. - I am sick today.

Estar is used to express an action in progress (progressive tense)

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.