What is the difference between the imperfect tense and the passé composé?

They are both used to talk about the past, however the imperfect tense talks about something that happened habitually or over a long period of time in the past and the passé composé is used to talk about a finished one-off action that happened in the past. For example: To say: "When I was younger I played tennis" you would use Imperfect as you played tennis multiple times when you were younger. "Quand j'étais plus jeune, je jouais au tennis"To say: "On Sunday I played tennis" you would use the passé composé as you only played tennis on Sunday as a one-off completed action. "Dimanche, j'ai joué au tennis"