What is the difference between the imperfetto and the passato prossimo?

The translation of the imperfect tense would be "I used to do/ I was doing". Hence, this tense would be used to describe an action in the past that had an unspecified starting or finishing time, it was continuous. i.e. When I was younger I used to go swimming/ I was swimming. One can also use the imperfect to express physical or emotional conditions that occurred in the past, the weather, age and time (all in the past). With the past tense, or passato prossimo, it refers to the translation "I did/ I have done". Hence, the past tense would be used to express an action that was in the past, but is finished now, and happened in a set time frame. i.e. Yesterday I went swimming.