SetEnv, SetEnvIf, and SetEnvIfNoCase directives conditionally set environment variables accessible by scripts and apache based on HTTP Headers, Variables, and Request information.
How to password-protect, Allow or Deny a visitor based on a condition. If you are having trouble getting htaccess-based password protection to work see: Troubleshooting htaccess Authentication: Getting it to work
Some good examples for how to use the Files and FilesMatch directives in .htaccess files and httpd.conf files for Apache.
<FilesMatch ".(htm|html|css|js|php)$"> AddDefaultCharset UTF-8 DefaultLanguage en-US </FilesMatch>
Learning about charset's and file types maybe pretty boring, but using .htaccess it can be fun! Here's a quicklist htaccess cheatsheet for adding the correct Charset to a web document.