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

Both 'ser' and 'estar' can be translated as 'to be'. So, when you need to use 'to be', you need to ask yourself a question to know which one to use:

Am I talking about the way an object is all the time (WHAT it is, the essence of it), or am I talking about the way an object is at a particular moment in time (HOW it is, the state that it is in)?

For the permanent essence of an object or person, including occupation (and for telling the time), we use SER.

For the current state of an object or person (including its location, however permanent that location may seem), we use ESTAR.

The difference between the two can be demonstrated using this example:

Ella está loca - She is acting crazily (implying that she is not actually crazy)

Ella es loca - She is crazy (implying that she is a crazy person).