This excerpt was taken from amazing book written by Linux advocate Evi Nemeth( unfortunately recently lost in the sea http://www.theregister.co.uk/2013/07/05/evi_nemeth_unix_dead/ ) in her one of the first and best linux admin book: Linux® Administration Handbook. For this case it was in the second edition of the book, I am not sure if this was duplicated in the latest 4th edition of the book, as by that time is NIS and NIS+ is less relevant.
Configuration information can be distributed in several ways. Every
system understands flat files and knows how to use DNS to look up
hostnames and Internet addresses. Most also understand NIS. Since a
given piece of information could come from several potential sources,
Linux provides a way for you to specify the sources that are to be
checked and the order in which the checks are made.
In the original (pre-Linux) implementation of NIS, some configuration
files (the /etc/passwd and /etc/group files in particular) had to be
configured to “invite in” the contents of the corresponding NIS maps.
The invitation was extended by inclusion of special incantations in
the files themselves. A lone “+” at the beginning of a line would
include the entire NIS map, “+@netgroup” would include only entries
relevant to a given netgroup, and “+name” would include a single
This approach was never very well liked, and it has been superseded by
a central config file, /etc/nsswitch.conf, that allows an explicit
search path to be specified for each type of administrative
information. The original behavior can be emulated by use of a
compatibility mode, but it’s unlikely you would want to use this
feature on a newly configured network. (Unfortunately, emulation is
most distributions’ default.)
The common sources are nis, nisplus, files, dns, and compat; they
refer to NIS, NIS+, vanilla flat files (ignoring tokens such as
“+”), DNS, and NISified flat files (honoring “+”), respectively.