is_IIS is part of the web server detection. It is set in [wordpress root folder]/wp-includes/vars.php in the following manner:
$is_IIS=strstr($_SERVER['SERVER_SOFTWARE'], 'Microsoft-IIS') ? 1 : 0;
A classic example of using the server detection is the pluggable function
wp_redirect function, which handles the difference between IIS and Apache redirects. Plugin and theme developers should similarly use it for web server specific features.
Back to full list of global variables.