сказать - to say/tell words/phrases/sentences/facts
рассказать - to tell a story in details

Here is a good explanation:

As for the differences between сказать and рассказать.
Basically, сказать is just to say something like a word for example.
Сказать слово/предложение

Рассказать is to tell something, dipping into details. Like:

Рассказать историю. Рассказать о себе

Here is a counterpart.

Скажи, что ты вчера делал vs. расскажи, что ты вчера делал.

The first one is like you're just asking for general information about what the person was doing/did yesterday. And the answer would be something like:

Сходил в кино.

The second one implies that the asker wants to know a more detailed version of what the other pesron was doing/did yesterday. The answer would be something like that:

Сходил в кино. Там такой фильм хороший показывали and blah blah blah.