What is the difference between the pretérito perfecto and the pretérito indefinido?

The pretérito perfecto (he llamado, has llamado, ha llamado..) is used to describe an action which happened in a time which is not yet past, or is still part of the present. For example this year, this week, today. Hoy he trabajado tres horas. This verb tense can also be used when we are talking about an action which took place in the past, but without referring to a specific moment, which can refer to experiences we have had or things we've done in our lives. No he probado nunca la comida mexicana.
The pretérito indefinido (llamé, llamaste, llamó..) is used to describe an action which took place and was completed in the past. This ca be a week ago, a month ago, last summer or last year. Ayer no trabajé. It can also be used without a specific time marker when it is obvious the action occurred in the past and has been completed. Fleming descubrió la penicillina por azar.