What is the difference between estar and ser?

Estar and ser both mean "to be" in Spanish. However, they are used differently. The basic difference is that estar is usually used to describe something temporary, whereas ser usually describes permanent states. Estar has the following uses: feelings, conditions, actions, locations, and positions. Feelings include sadness, happiness, or excitement; a condition might be being tired or ill "estoy cansado/a, estoy enformo/a"; estar is used to form the gerund - to be doing something = estar haciendo algo; estar is used to describe a location - where something or someone is - "estoy en el supermercado" (I am at the supermarket), even for locations that are permanent "La Torre Eiffel está en París" (the Eiffel Tower is in Paris); lastly positions, for example, being seated = estar sentado.

Ser is commonly used in the following situations: descriptions (physical or describing personality), occupations, time, origin, events, and relationships. When you describe someone as a person (not how they are currently feeling) you use ser - "ella es alta, delgada y muy inteligente" (she is tall, slim and very intelligent), this also applies to their job "ella es profesora" (she is a teacher); ser is always used to say what time it is "son las cinco menos cuarto" (it's quarter to five); origin - where you are takes estar, but where you're from takes ser - "soy de Inglaterra" (I am from England); events are an exception to the location rule - for an event you use ser "la reunión es en el ayuntamiento" (the meeting is in the town hall); last but not least relationships always take ser, for example "él es mi padre" (he is my father).

This may seem like a lot to remember, but here's a tip: if to be is followed by a noun you will always need to use ser!