Apache documentation on .htaccess
Apache documentation on auth.html

to create a new password file:

# htpasswd -c /var/www/passwd/passwords rbowen
New password: mypassword
Re-type new password: mypassword
Adding password for user rbowen

to append to existing password file:

# htpasswd /var/www/passwd/passwords rbowen
New password: mypassword
Re-type new password: mypassword
Adding password for user rbowen