Soy (ser): for invariable conditions, like nationality, occupation, how tall you are...etc. - De donte eres? ( Where are you from? ) - Soy espanol. / Soy espanola. ( I am Spanish. ) - Que haces?/ A que te dedicas? ( What do you do?) - Soy camarero. / Soy camarera. ( I am a waiter. / I am a waitress.) - Quien es? ( Who is it? ) - Es un cantante. Es alto y delgado. ( He is a singer. He is tall and thin.) ...etc Estar: for variable or temporary conditions, like mood...etc. - Donte esta Barcelona? ( Where is Barcelona? ) - Esta en el noreste de Espana. ( It is in the northeast of Spain. ) - Donte esta el perro? ( Where is the dog? ) - Esta debajo del sofa. ( The dog is under the sofa. ) * Estar for the mood, it will put an adjective after estar. - Que te pasa? ( What's wrong? ) - Estoy cansado. / Estoy cansada. ( I am tired. ) - Estoy enfermo. / Estoy enferma. ( I am sick. ) - Estoy preocupado. / Estoy preocupada. ( I am worried. ) - Estoy nervioso. / Estoy nerviosa. ( I am nervous. ) - Esroy triste. ( I am thirsty. ) ...etc.

Soy is for permanent conditions and estar is for temporary conditions. That's the main difference. For example: Soy estadounidense. ("I am American.") Your nationality doesn't change. Estoy contenta ("I am content.") Your mood changes. Estoy en mi casa. ("I am in my house.") You can leave your location (house)...so it's temporary. Remember this phrase: HOW YOU FEEL AND WHERE YOU ARE, ALWAYS USE THE VERB ESTAR. It's a rhyme that I memorized in Spanish class to help me remember when to use estar :P

Hi, the greatest difference between them is: "soy" in Spanish is the same that "sou" in portuguese, both means the expression "I am" or "You are" or "he is" in english, but the principal difference is that this term only can used to say things about the people are. For example: I am handsome. Or "You are poor!". Or "She is ugly". Or "We are rich." "estoy" in Spanish is the same that "estou" in portuguese, both means the expression "I am" or "You are" or "they are" in english, but the principal difference is that this term only can used to say the place where the peoples are or how they are feeling. For instance: I am at home. Or "You're bored". Or "They are at the church." Or "We are happy". I wait to helped you. Any doubt, write me.

it is not simple to say in short. But "estar" is used for location and "ser" for personal characteristics. These are the more basic functions, but there are some others. ex: "estoy en casa" and "soy alto".