https://www.garron.me/en/go2linux/useradd-vs-adduser-ubuntu-linux.html
https://www.google.com/search?q=0-web.ru+useradd&ie=utf-8&oe=utf-8#q=debian+useradd+adduser
http://linuxers.org/article/differences-between-useradd-and-adduser-commands
http://unix.stackexchange.com/questions/121071/what-does-adduser-do-that-useradd-doesnt
useradd is a low level utility for adding users. On Debian, administrators should usually use adduser(8) instead.