.nfs files: what are they?

The .nfsXXXXXX files are created when an open file is removed but is still being accessed. The NFS client trying to maintain proper "delete on later close" unix behavior within its own operational abilities. This NFS behavior is known as "silly rename": http://nfs.sourceforge.net/#faq_d2

NFSv4.1 will get away from this behavior with OPEN4_RESULT_PRESERVE_UNLINKED: http://tools.ietf.org/html/rfc5661#section-18.16.