Emergent behaviors in simulations require explanation, so that valid behaviors can be separated from design or coding errors. We present a taxonomy, to be applied to emergent behaviors of unknown validity. Our goal is to facilitate the explanation process. Once a user identifies an emergent behavior as a certain type within our taxonomy, exploration can commence in a manner befitting that type. Exploration based on type supports narrowing of possibilities and suggests exploration methods, thus facilitating the exploration process. Ideally, a taxonomy would be robust, allowing reasonable variation in behavior type assignment without penalty in cost or correctness during the exploration process. The taxonomy we present is robust, comprehensive and suitable for use with our established emergent behavior exploration methods. In addition to the taxonomy, we present our design rationale, and a summary of results from a test application of our taxonomy.