Simply, the imperfect tense is used to convey an action in the past that was taking place over a period of time e.g. 'Je regardais la télé' = 'I was watching TV' - the act of watching TV was not instantaneous. On the other hand, the perfect tense is used to convey a finished action in the past e.g. 'J'ai vu Paul' = 'I saw Paul' - the act of catching sight of Paul IS instantaneous and completed immediately.