In English, l'Imparfait tends to be the preterite or the past continuous. In French, it's used for long action (still occurring or not), description or story-telling. It's also more formal.
Le passé composé isn't use in French formal literature or texts, and that's the English equivalent of present perfect.
L'Imparfait is alike of his English equivalent because they approximately have the same value. But that's not the case for the passé composé, this is not used for assessment like in English.
Be careful, the preterite can also be Passé Simple in French.
