Nomenclatures differ, but, generally: The passive voice is "ser" + participle. The the passive meaning is obtained using the passive voice, the passive "se", the passive third-person plural, the passive "tú", and by shifting the object to the head of the sentence. But to avoid writing a book, I'll stick with "ser" + participle and "estar" + participle. Generally, "ser" + participle describes an action, while "estar" + participle describes the state that results from the action.
La casa es vendida a menudo.
The house is sold a lot. [habitual action]
La casa fue vendida el mes pasado.
The house was sold last month. [past action]
¡La casa por fin está vendida!
The house is finally sold! [It was on the market for months.]
La casa estuvo vendida.
[This case is a bit odd and I'll pass over it.]
La casa estaba vendida.
The house was sold.
In the last example, I walked by the house yesterday and the "FOR SALE" sign had a "SOLD!" banner taped to it, indicating the new state of the house. I have no idea when the sale happened.
Notice, by the way, that there is no "estar" passive meaning. In the
above examples, "estar" functions in its usual way, and "vendida"
functions purely as an adjective.