If you allow me, I have a couple of corrections to Gustavo's answer:
1.- Fue is not pretérito perfecto. its pretérito indefinido. The 'pretérito perfecto' for the verb 'ser' (conjugated for third person of the plural: ellos), is 'han sido'.
2.- Then, to give an example of the 'preterito indefinido', he uses the sentence 'el fue un buen alumno'. Well, I have to say this is not quite correct. Normaly when we want to describe characteristics of a person in the past, we use the 'preterito imperfecto', most of the time when you talk about how a person was, you will find the verb 'era': Era una buena persona. Era muy discreto. Era simpático. There are some exceptions for this, for example if the person is dead or if we talk about long ago (or both). Example: Miguel de Cervantes Saavedra fue un novelista, poeta y dramaturgo español.
3. Then, the example "Mientras leía el periódico, me acordé de que tenía examen", has both, indefinido and imperfecto. So lets put clear that 'Leía' and 'Tenía' are in imperfecto (they had some duration for a while), but 'me acordé' is certainly indefinido. I would use this example to clarify one of the most common uses of the 'pretérito indefinido': actions in the past that interrupted an action that was already taking place.
4. Actions in 'pretérito imperfecto', are always finished, like if you say you used to go to the cinema every weekend, its clear you don't do it any more.
REF.: I'm a Spanish Teacher.