What is the difference between 'ser' and 'estar'?

First of all, this is a great question and something which almost every English speaker struggles with when learning Spanish because we only use one verb to convey both uses of 'ser' and 'estar'. The main thing to remember is that 'ser' is often used for permanent traits and characteristics of a person or object and to describe the origin of something or someone. 'Estar' is often used when talking about temporary states or qualities about a person, such as feelings, about an object or location. The best way to practice this is through examples and thinking about the implied or assumed meaning of a sentence. Practise examples and you'll understand it in no time! Think about the difference between, "The boys are very handsome in those suits" and "She is a very beautiful woman". The implication is that the boys are looking handsome in this particular outfit, on a particular day. Whereas, the woman is assumed to be permanently beautiful. Therefore, we say, "Los chicos están muy guapos en esos trajes" and "Ella es una mujer muy guapa". Here's another example. "The Sagrada Familia is in Barcelona" and "My mum is from Yorkshire". The current location of the Sagrada Familia can change. However, the fact that the mum is from Yorkshire cannot change. So, we say, "La Sagrada Familia está en Barcelona" and "Mi madre es de Yorkshire". Here's another one, "He is a very happy man" and "I am a bit sad right now". The man's personality trait of being happy is considered permanent. Whereas, the emotion of feeling sad in the moment is subject to change. So, we can say, "El es un hombre muy feliz" and "Estoy un poco triste ahora mismo". Finally, we can consider 'ser of event'. This is when we use 'ser', instead of 'estar' to talk about the location or time of an event, such as a party or meeting. E.g. "The class is in the library at 4pm" = "la clase es en la biblioteca a las 4".