A chimichanga is simply a burrito that has been deep fried. Burritos are made by filling large tortillas and folding over all sides, leaving no open ends. Chimichangas are made by following this same pattern, but adding a step by deep frying the burrito before serving.
Burritos and chimichangas can have a variety of fillings, including beans and meat. The beans and meat are often mixed with cheese before being wrapped in a large tortilla. What sets burritos and chimichangas apart from regular tacos is the method of folding the tortilla. All ends of the tortilla are folded over to keep the filling from escaping during frying or eating.