Loosely defined, it's various chips that implement simplified computational models of neurons, and some plasticity functions. They are usually simplified becaus

Loosely defined, it's various chips that implement simplified computational models of neurons, and some plasticity functions. They are usually simplified because going into the greatest detail (modeling hodgkin-huxley type channels) would require too much computation. In the neuroscience community there is not yet an acceptable model for a simplified neuron so everyone picks some spiking neuron model or they make up one. Even less is known for plasticity functions. I believe the idea is that if you simulate too many of them, something useful will happen.