What is the difference between the preterite and imperfect tenses?

Imagine a long line stretching from the present moment all the way back into the past. We can explain these two tenses by the way we mark them on the line. The preterite tense, generally representing past actions that are understood as completed, will be represented by an 'x' on this line. An example of this might be 'fui al cine anoche,' because you are describing something you did at a specific time. The imperfect tense is different, describing an action that took place over time or regularly over time - this will be represented by brackets which extend over a section of the timeline. An example of this might be 'Cuando era niño, jugaba al fútbol,' because you played football regularly over a longer period of time when you were little.