The file uses the following format:
The hostname is the name of a host which is logically equivalent to the local host. Users logged into that host are allowed to access like-named user accounts on the local host without supplying a password. The hostname may be (optionally) preceded by a plus (+) sign. If the plus sign is used alone it allows any host to access your system. You can explicitly deny access to a host by preceding the hostname by a minus (-) sign. Users from that host must always supply a password. For security reasons you should always use the FQDN of the hostname and not the short hostname.
The username entry grants a specific user access to all user accounts (except root) without supplying a password. That means the user is NOT restricted to like-named accounts. The username may be (optionally) preceded by a plus (+) sign. You can also explicitly deny access to a specific user by preceding the username with a minus (-) sign. This says that the user is not trusted no matter what other entries for that host exist.
Netgroups can be specified by preceding the netgroup by an @ sign.
Be extremely careful when using the plus (+) sign. A simple typographical error could result in a standalone plus sign. A standalone plus sign is a wildcard character that means "any host"!
Modern systems use the Pluggable Authentication Modules library (PAM). With PAM a standalone plus sign is only considered a wildcard character which means "any host" when the word promiscuous is added to the auth component line in your PAM file for the particular service (e.g., rlogin).