What is the difference between the two Spanish verbs for 'to be': 'ser' and 'estar'?

There are many specific purposes that require the use of either 'ser' or 'estar', but in the simplest terms there is one key difference. 'Ser' refers to 'being' in the sense of existence, describing what things are like in a relatively permanent sense. 'Estar', on the other hand, refers to 'being' in the sense of position and place, describing temporary states and usually resulting from actions or events. That being said, there are a few exceptions to this rule that must simply be learnt, such as the use of 'ser' to tell the time, which is not a permanent concept. However, the rule applies to the vast majority of situations that require the verb 'to be' in Spanish.