The imparfait is used to describe the past.
The passé composé is used to narrate individual events in the past.

Novels and stories use the imparfait to set the scene with a description.

Un jour d'automne, un jeune homme marchait dans la rue. Le ciel était gris et il pleuvait légèrement. Le jeune homme était grand et il portait un gros chapeau. Il marchait lentement. Il était triste.

One Autumn day, a young man was walking in the street. The sky was grey and it was raining lightly. The young man was tall and he was wearing a large hat. He walked slowly. He was sad.

Novels and stories use the passé composé to tell you what happened.

Tout d'un coup, le jeune homme a glissé sur une peau de banane. Son chapeau est tombé sur le trottoir. Quatre enfants ont commencé à rire.

Suddenly, the young man slipped on a banana skin. His hat fell onto the sidewalk. Four children began laughing.