.htaccess (dot-htaccess) is the directory-level configuration file of the Apache web server. It has a lot of control over how your web server works. Here are some tutorials about it.

• Wikipedia entry of .htaccess
• htaccess-guide.com
• Apache tutorial
• Example usage of .htaccess

For example, the following in the .htaccess file

<Files ~ "\.(inc|class)$">
Deny from all
</Files>

will make sure that people can not browse inc and class files.

And ...

IndexIgnore *

will prevent browser from directory listing your files on your server.