What is the difference between the Prétérito Perfeito Simples (PPS) and the Imperfeito do Indicativo?

To begin, the first rule to memorize is that both the PPS and the Imperfeito do Indicativo are used in order to narrate an action that happened in the past, that has already happened. However, this similarity is what creates confusion between the two tenses. Although they both narrate past actions, the difference lies within what type of action is narrated. 

- Pretérito Perfeito Simples (PPS): expresses and narrates an action that took place and was completed in the past. 

For example:

Ontem fui ao cinema -> Yesterday I went to the cinema (By going to the cinema and watching a film, the narrator undertook and completed the action in the past)

- Imperfeito do Indicativo: is used in order to narrate repeated or habitual actions which occured in the past (this tense is most frequently used when describing events).

For example:

No ano passado, eu comia todos os dias um iogurte ao pequeno almoço -> Last year, I ate a yogurt every day for breakfast. (The imperfect is used here in order to describe an action that happened frequently and repeatedly in the past.)