What's the difference between ser and estar?

Ser is used in reference to things that are permanent, or unchanging - eg. Soy de Inglaterra. You can't change where you're from, so you need to use ser. Estar is used to describe temporary things - eg. Estoy en Barcelona para mis vacaciones. Unfortunately, holidays don't last forever, so you need to use estar to convey this. Estar is also used to describe emotion and location.