passwd

Modify a user password.
First, the user is prompted for their current password. If the current password is correctly typed, a new password is requested. The new password must be entered twice to avoid typing errors.

Syntax
      passwd [-i infosystem] [-l location] [name]

Options
       -i infosystem
	   This option specifies where the password update should be applied.
	   Under Mac OS X 10.3, supported infosystems are:

	   netinfo
		 (default) The netinfo database containing the user's pass-
		 word.	If no -l option is specified, the local netinfo
		 database is assumed.

	   file	 The local flat-files (included for legacy configurations).

	   nis	 A remote NIS server containing the user's password.

	   opendirectory
		 A system conforming to opendirectory APIs and supporting
		 updates (including LDAP, netinfo, etc).

     -l location
	   This option causes the password to be updated in the given location
	   of the choosen infosystem.  When changing only the local password,
	   pwd_mkdb(8) is used to update the password databases.

	   for netinfo,
		 location may be a domain name or server/tag

	   for file,
		 location may be a file name (/etc/master.passwd is the
		 default)

	   for nis,
		 location may be a NIS domainname

	   for opendirectory,
		 location may be a directory node name

	   The super-user privilages are not required change a user's current
	   password if only the local password is modified.

The new password should be at least six characters long and not purely alphabetic. Its total length should be less than _PASSWORD_LEN (currently 128 characters) although some infosystems allow longer passwords.

Numbers, upper case letters and meta characters are encouraged. Once the password has been verified, passwd communicates the new password information to the authenticating host.

Files
/etc/master.passwd The user database
/etc/passwd A Version 7 format password file
/etc/passwd.XXXXXX Temporary copy of the password file

"Chico : you can't come in unless you give the password...
Well, what is the password?... I got it! Haddock! - Groucho Marx

Related:

passwd man page - Apple.com
openssl passwd - compute the hash of a password
chgrp - Change group ownership
chmod - Change access permissions
chown - Change file owner and group
quota - Display disk usage and limits
who - Print who is currently logged in
chpass(1)
login - log into the computer
Password generator
kb/HT1352 - Firmware password protection in OS X