The methods Windows and NFS use to represent user and group identities are different and are not necessarily directly interchangeable. Identity Mapping is the process of converting from an NFS identity representation to a Windows representation and vice-versa.
This blog post describes the selection, configuration and usage of the user and group identity mapping options. The NFS client is available selected versions of Windows 8 and Windows Server 2012. This post will assist systems administrators when installing and configuring the NFS components within Windows 8 and Windows Server 2012. It describes the available mapping mechanisms and provides guidance on which of those mechanisms to use in common scenarios.
See the entire post at http://blogs.technet.com/b/filecab/archive/2012/10/09/nfs-identity-mapping-in-windows-server-2012.aspx.