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

This is a tricky one. The rhyme I learned "how you feel and where you are - that is when you use estar!" is pretty useful, but I like to remember it based on whether the state of something can change. So your personality, the colour of a pillow uses "ser" because they are unchanging, whereas your mood, where something is in a room uses estar because it describes a state of something which may be changed.