Many times they are interchangeable ... "I'm thinking about that time we went to the beach." "I'm thinking of that time we went to the beach." But "thinking about" can imply you are putting more effort into the thought or the thought is more meaningful, as opposed to "thinking of" something. "I'm thinking ABOUT how much that costs." "I'm thinking OF a number between one and ten."

One occasion in which they are NOT interchangeable: arriving at the answer to a question. "Question number 7 was difficult. Though I remember studying it, I could not think OF the answer." "Think ABOUT the answer" would be incorrect here.