Both are conjugated forms of the verb "ser" (although 'fue' can also be a form of "ir"), which is a highly irregular verb. They are both used to describe things that "were" (ie., in the past). The difference is in the tense being used: "era" is imperfect, while "fue" is preterite.
The imperfect is typically used for habitual actions (in the past) and actions that were interrupted. Preterite is used normally for actions that were *finished* in the past, or actions that interrupted another one. There are more uses to each one, but I'd suggest checking a grammar for that.
"*Era* casi de noche cuando se *fue* a casa" It *was* almost night when he *went* home"
In this case I used "fue" as conjugation of "ir", not "ser", but it works for the example.
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