What's the difference between imperfect and passé composé?

Passé composé is a past tense used to describe a completed action. For example, I went to the cinema (Je suis allé(e) au cinéma).

Imperfect is also a past tense. This time it is used to talk about something you used to do regularly. For example, when I was younger I swam (Quand j'étais plus jeune je faisais de la natation). Imperfect can also be used to describe scenes.