Ser is permanent and Estar isn't. We had this rhyme in my class "how you're feeling, where you are, that is when you use Estar". That really helped me with learning the difference!

Well, "eres" is the second person singular of the simple present of the verb "ser (to be)". If you want to say "you are very smart", you have to say "tú eres muy inteligente". "Somos" is the first person plural of the same verb ("ser") and time (simple present) as "eres". "We are beautiful" is "nosotros somos muy hermosos", "we are happy" is "somos muy felices". There is one peculiarity about "somos": when you use a third person plural subject you can use either the verb in the third person or in the first person plural. If you use the third person you are saying that you are not included in that subject. If you use the first person plural you are including yourself in the cathegory that the subject represents. For example: "Spanish people are very nice"= "los españoles SON (third pers. plu.) personas muy majas" or " los españoles SOMOS personas muy majas". In the first case, you are not considering yourself Spanish, maybe because you are English or because you don't want to put yourself in that cathegory of Spanish people (the nice one). In the second case, you are including yourself in that cathegory because you are Spanish or because you fell like you are one of them. In English you could translate the second case with "we Spanish are very nice people".