When do we use the 'passé composé' and 'imparfait' in French?

Both tenses refer to actions in the past. With the passé composé, we use this to describe an action which was completed in the past at a specific moment.
For example: Je suis tombé (I fell).
We use the imparfait to describe habitual (was doing/used to do) or background actions in the past, or to set the scene in a description
For example: Quand j'étais petit (background action), je jouais (habitual) souvent avec mon frèreWhen I was young, I used to play often with my brother.
Look at both tenses used in this sentence to see how they are used:
Pendant que je regardais la télé, mon portable a sonnéWhile I was watching TV, my phone rang