.htpasswd
File used to store user names and passwords, used in conjunction with the .htaccess file. Below is a basic example of how to create a .htpasswd file.
htpasswd -c .htpasswd joe
The above command would create a file named .htpasswd with the username joe. Once created you will be prompted to enter the password for joe twice.
Generally, the .htpasswd or the ht password file must be correctly attributed to be accessed; generally set to a group with read access. *NIX users should chmod 644 the file.
Also see: .htaccess