"Eu sou" means "I am" for permanent or lasting attributes. Situations to use the verb "Ser": - Time: "Que horas são? São 5 horas" [What time is it? It's 5 o'clock] - Day/Date: "Hoje é 8 de janeiro" [Today is 8th january] - Nationality: "Eu sou brasileiro" [I am Brazilian] - Profession: "Eu sou médico" [I am a doctor] - Marital Status: "Eu sou solteiro" [I am single] - Relationship: "Ela é minha mãe" [She is my mother] - Inner Characteristics/Features: "Eu sou alto" [I am tall] - Possession/Ownership: "Este carro é meu" [This car is mine] "Eu estou" means "I am" for temporary states or locations. Situations to use the verb "Estar": - Location: "Eu estou na casa de Carlos" [I am at Carlos' house] - Position: "Ela está sentada" [She is sitting] - Actions: "Eu estou tocando violão" [I am playing the guitar] - Condition: "Eu estou cansado" [I am tired] - Feeling: "Eu estou triste" [I am sad]

http://www.easyportuguese.com/portuguese-lessons/to-be/. this link explain very well about ser and estar.

An easy way to understand is you imagine a dialogue, you and Barack Obama in a dialogue, get this: Barack Obama - Hello! Who are you? You (Gianni) - Hello! I am Gianni! (EU SOU o Gianni) Now let's assume that you are talking to Barack Obama by phone: Barack Obama - Hello, Gianni! Where are you now? You - Well ... Now I'm in my house, eating dinner. (Bem... Agora EU ESTOU em minha casa, jantando.) "EU ESTOU" means a context, a place where you are, a momentary feeling, something that is not permanent, can be hunger, fear. "EU SOU", is something that is tied to you, who you are, is something more permanent.

Depends of context too .. for example: I am Talita. Eu SOU Talita. ( verb Ser ) I am in the beach. Eu ESTOU na praia. ( verb Estar )

Pense que: Ser é um estado permanente . Exemplo: Eu sou Eliana ou João é médico. Estar é um estado passageiro. Exemplo: Eu estou aqui ou João está lindo.