Apparently the difference depends on the linux distribution.
On gentoo, I emerged app-admin/superadduser
but I still don't have adduser:
$ which adduser
which: no adduser in (/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/opt/bin:/usr/x86_64-pc-linux-gnu/gcc-bin/4.9.3)
