Some interchangeability here..but the way it was splained to me speaking in general terms..Cajun is rural country type food. Creole is a more sophisticated culinary style exemplified by the efforts of various ethnic groups who settled around Nor'leans. One test I use is the use of tomato products. For example if you see a gumbo recipe calling for tomatoes it would generally be classified as Creole..while shunning tomatoes is a clue that it should be Cajun. Here is an link that might tell it better.

About Louisiana cuisine