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

The passato prossimo is used for completed actions. Oggi ho mangiato una mela-Today I ate an apple. Whereas the imperfetto is used for repeated or descriptive actions. Quando ero piccolo andavo a scuola (repeated)-When I was little I went to school. Ieri faceva caldo (descriptive)- Yesterday it was hot.