I will give a short introduction in order to explain the difference, right?
John is beautiful -> Indicates essence: John is beautiful today, John will be beautiful tomorrow.
John is beautiful today -> Indicates something that may to be passenger: John is beautiful today, but may not be beautiful tomorrow.
In the above case the verb "to be" indicates essence and state.
* state = something that can to be passenger
In the Portuguese, don't exist a verb to indicate essence and state. Different verbs are used: the "to be" in Portuguese is the verb "ser" and is the verb "estar"
The verb "ser" indicates essence.
The verb "estar" indicates state.
Examples:
Verb "ser"
João é bonito (John is beautiful) -> Indicates essence: John is beautiful today, John will be beautiful tomorrow.
* "é" is the conjugation of the verb "ser" (essence) in present tense
Verb "estar"
João está bonito (John is beautiful) -> Indicates something that may to be passenger: John is beautiful today, but may not be beautiful tomorrow.
* "está" is the conjugation of the verb "estar" (state) in present tense
It "Era" or "Estava"?
"Era" is the conjugation of the verb "ser" (essence) in past tense.
"Estava" is the conjugation of the verb "estar" (state) in past tense.
* As the verb "to be" can be translated as "ser" or "estar" in present tense, so,
"era" or "estava" can be translated as the verb "to be" in past tense.
I have helped you :)