Expert system
Case-based expert systems have a number of cases; each case describes the problem in context, and  the solution of the problem, in the specific case. The current problem is matched as closely as possible to a case. The solution of the case found is then applied to the current problem. The main problem of such systems is to define how cases relate to each other, or are similar. An example of where such a system may be used is that of a patient: This patient shows a given number of symptoms, and the expert system could diagnose the patient.