Hypertext Access, commonly shortened to htaccess, They are placed inside the web tree, and are able to override a subset of the server's global configuration for the directory that they are in, and all sub-directories. For more information check this useful resources http://en.wikipedia.org/wiki/Htaccess http://www.htaccessbasics.com/
an htaccess file can do many things. redirect, handle errors, write custom url's, block access to users, authenticate users, hide webpages, etc.....