What is the difference between the preterite and the imperfect in Spanish

Spanish has two past tenses, the preterite and the imperfect. The difference between them can be hard to grasp. Generally speaking the preterite is used for actions in the past that are seen as completed. For example "Pablo jugó de las una hasta los dos." This would be a completed action as it has a clear beginning and end. On the other hand the imperfect is used for actions in the past that are not seen as completed i.e. there is no definite beginning or end. It is also used to describe habitual actions. For example "Yo caminaba por el parque cada domingo".