What is the difference between le passé composé and l'imparfait?

Le passé composé is used for completed actions in the past, whereas l'imparfait is used for either ongoing/habitual actions in the past or for general description. For example, if you were to say "Yesterday I woke up at nine o'clock", you would use le passé composé in French because it contains a completed one-off action in the past. However, if you wanted to say "When I was little, I woke up every day at nine o'clock," you would use l'imparfait as it is a habitual action in the past, as shown by the phrases 'When I was little' and 'every day'. Equally, you would use l'imparfait for general description, e.g. "It was hot"; "the students were noisy". This is because it describes a general state of being rather than a specific completed action.