NIS is a bit of an old-school solution on UNIX systems for sharing user information, including logins, across systems. This allows administrators to centralize all user account information, as well as home directories, across any number of physical machines. There are some security implications (like having to disable iptables on the server, see below), and maybe LDAP would be a better choice. But for a home or small organization behind a good firewall, an NIS system could be a lot simpler to create.