What's the difference between passé composé and l'imparfait?

Passé composé is mostly used to describe actions that had a definite end to them. For example, if you want to say that yesterday you ate breakfast, met a friend and saw a film you will use passé composé to describe these actions (j'ai mangé le petit déjeuner, j'ai rencontré un ami et j'ai regardé un film) because they all had a definite end to them.L'imparfait, on the other hand, is used to describe the scenery (e.g. Il pleuvait - It was raining), the time (e.g. Quand j'étais jeune - When I was young) or to describe a continuous or an ongoing action that is still happening now but began in the past (e.g. Quand je l'ai vu, il lisait un livre. - When I saw him he was reading a book (and probably is still reading it now).