What is the difference between the preterite and the imperfect past tense?

The preterite tense is used when we are describing something that happened once in the past with a given time, for example: 'I picked up the phone last Tuesday when he called me'. The imperfect tense is used to describe an action that was ongoing in the past with no given time frame, for example: 'When I was younger, I used to play piano'. A sentence which would include would be 'I was cooking when the phone rang'. I was cooking would be translated in the imperfect tense and the phone that rang would be translated into the preterite tense.