What is the difference between `register_buffer` and `register_parameter` of `nn.Module`

I was reading the code of mask-rcnn to see how they fix their bn parameters. I notice that they use self.register_buffer to create the weight and bias, while, in the pytorch BN definition, self.register_parameter is used when affine=True. Could I simply think that buffer and parameter have everything in common except that buffer will neglect the operations to compute grad and update its values ? B