Windows Internet Naming Service. essentially it performs the same function as local LMHOSTS files for resolving NetBIOS names to IP addresses except that it gives you management at one location. As far as security it puts all your eggs into one basket so to speak, so all you have to do is protect the one server rather then protect the individual LMHOSTS files on all machines.

The KB article Q150800 might give you better insight as to the differences.