What's the difference between these two? According to wiktionary both are perfective forms of говорит but I'm not sure if they're really interchangeable. When should each one be used?
Сказать means to make one or more statements which have the effect of informing the listener on some clearly identified subject. For example:
Скажите что я позвонил.
In contrast поговорить means to have a talk with someone. It conveys the idea of uttering appropriate words in order to revolve a situation or to make one's position clear. For example:
Наш сын меня не слушает. Ты должен с ним поговорить.
Хорошо, я поговорю.