The ‘useradd’ command in linux is a popular tool used to create or add user accounts.Much like the command ‘usermod’, the ‘useradd’ command affects many files when it is invoked to create a new user account, namely, /etc/shadow, /etc/passwd, /etc/gshadow , /etc/login.defs, /etc/group

Source: LXer – How to create a user in Ubuntu/Debian: ‘useradd’ command usage, a beginner’s guide