useradd/userdel/usermod

Used to add/delete/modify users in /etc/passwd. These commands also perform various other convenience functions. See man pages for more information.

pwconv/grpconv

Used to convert passwd and group files to "new-style" shadow passwords. Virtually all Linux systems already use shadow passwords, so you should never need to use these commands.

pwunconv/grpunconv

Used to convert passwd, shadow and group files to "old-style" non-shadow passwords. You should never need to use these commands.