SPANISH: Imperfect vs. Preterite tenses?

For Spanish cl, we are writing obituaries, and need to talk about the lives of the deceased. In the examples, my teacher instructs us to use "era" and "ten�a" so does that mean that all the verbs should be conjugated in the imperfect tense, not the preterite? But I also read some biographies online which used "fue", so I'm really not sure. Please explain the difference?