Perfect tenses....

These are both tenses you would use to talk about the past. You would use the imperfect to talk about something which went on for a long time, or for something which happened often. So you would say 'Je jouais au hockey quand j'avais treize ans', or 'j'allais souvent au cinéma'. You would use the perfect to talk about something which happened once. So, you would say 'Je suis allée ? Edimbourg avec Sarah samedi', or 'Mon prof a changé la couleur de ses cheveux', because these are things that happened only once.