What is the difference between ser and estar?

Describing people
Unlike english, Spanish has two verbs for "to be": ser and estar. As a general rule ser  is used to describe or answer questions about who or what something is, while estar describes how or where something is. Another way of saying this would be to say that ser indicates a permanent feature and estar indicates a temporary condition. 
Compare:
-"Está borracho" = he is drunk [temporary conition]
 - "Es borracho"= he is a drunkard [permanent feautre, a description of character, not state]

NOTE: Common exeptions to this include the adjectives pobre, rico, feliz, infeliz, inocente, culpable (poor, rich, happy, unhappy, innocent, guilty) which are all almost always preceded by ser when used to describe people. 

Jobs and occupations always go with ser 
e.g. "Soy médico"   "I am a doctor"

       "Es abogado"  "He is a lawyer"

Location

Estar is used to describe the location of objects. "El libro está en el estante" = "The book is on the shelf.
However, ser must be used for the location of events as opposed to people or things "¿Dónde es la fiesta?"