[root@larrywen opt]# type adduser 
adduser is /usr/sbin/adduser 
[root@larrywen opt]# type useradd 
useradd is /usr/sbin/useradd 
[root@larrywen opt]# which useradd 
/usr/sbin/useradd 
[root@larrywen opt]# which adduser 
/usr/sbin/adduser 
[root@larrywen opt]# ls -l /usr/sbin/adduser /usr/sbin/useradd 
lrwxrwxrwx. 1 root root   7 Jul 21 14:11 /usr/sbin/adduser -> useradd 
-rwxr-x---. 1 root root 97040 Feb 24 2011 /usr/sbin/useradd

[root@larrywen /]# useradd zhink 
[root@larrywen /]# id zhink 
uid=501(zhink) gid=502(zhink) groups=502(zhink)