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

Both 'ser' and 'estar' mean 'to be' but they are used in different contexts.'Ser' is used to identify and define a person or thing, so it says what somebody is, rather than what somebody is like, e.g. Ella es una mujer. It also refers to nationality (Ella es española), professions (Pedro es profesor) and characteristics of a person (inteligente/rubia). One good way of knowing when to use 'ser' is to ask yourself whether you could put an equals sign between the two parts of the sentence, e.g. Laura es inglesa (Laura = inglesa). 'Estar' is used to indicate something temporary, for example an illness or mood (Estoy may sana) or to describe what somebody or someone is like. This is something that could change in the future. It is also used to describe the location of something or someone, e.g. Estoy en Londres. Finally, it can be used to describe an action which is in progress, e.g. Está lloviendo (It's raining).