NFS Question

I have a question about the difference between the Debian packages nfs-user-server and nfs-kernel-server. I have been studying several HOWTOs on nfs, and I think I understand how to install it/set it up, but I am not completely clear on the difference between these two versions of the packages. nfs-user-server runs entirely in user space, but which package should I prefer? Or perhaps, which underlying misunderstanding do I have that prevents me from knowing which package I should prefer? (I notice that most of the info on nfs on the web seems to tell me about the nfs-kernel-server package.)

Thanks for any guidance you may give me!