Ser and estar both mean "to be". BUT: Ser is used to indicate an attribute or an inherent quality that is not likely to change. Estar is used to indicate a condition which is likely to change or it is not going to last forever. examples: 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. ( source: http://www.123teachme.com/learn_spanish/verbs_ser_estar )

Ser: is used when something has a permanent state during the time, or lifespan Example: Soy chileno (I am Chilean) Here we are talking about something that is like and "eternal" or long term feature on the subject. Therefore is used when you want to say something or someone has a long term feature (on the basis of the time such feature will remain), or such feature has always been present on such element, Estar: 1. Is used when you refer to a transitory feature, this means something is momentary and non eternal in terms of time, hence its use with the gerund in Spanish. (estar comiendo) Example: Estoy contento 2. Is used for physical location, like when you say "I am in the kitchen" Example: Estoy en la cocina RESTRICTIONS: Some adjectives, which have a time-related restrictions cannot be used with "ser", this is because such adjectives have the idea of "short time experience" attached to them, like "contento". You will master this with time. Apart from that, some regions in both Spain and Latin America, sometimes mix the used of these verbs, so do not worry if you hear something odd.

Estar is used to tell about the place of the things. For example: Donde esta mi libro? (Where is my book?)