A hot-linking is when some other site uses images hosted on yours. For example a.com has some pretty nice images. Then b.com decides that instead of hosting these images on their server, they can just link from their pages to the images hosted on site a.com.
Hot-linking usually is bandwidth and of course content stealing. The b.com site will not pay for the traffic used as the image is being loaded from site a.com.

So it is a good practice to prevent images hot-linking:

You can prevent the hot-linking of your images by creating a .htaccess file with the following content:

RewriteEngine onRewriteCond %{HTTP_REFERER} !^$RewriteCond %{HTTP_REFERER} !^http://(www.)?your-domain.com/.*$ RewriteRule \.(gif|jpe?g|png)$ - The above code will result in a broken image to be displayed when it is hot-linked.

The example above works for .gif,.jpg and .png files, but you can add any file extension.

If you place the .htaccess file in the main folder of your site it will disable hotlinking for all your site.

To block other type of files, just add their extension to the list above. For example to block movie files:

RewriteRule \.(mov|avi|wmv|mpe?g)$ - The Hot-Linking prevention is based on an Apache module called ModRewrite. So your web host should support it in order for you to be able to use these on your site.

We will be discussing ModRewrite in a separate topic.