Files that are loaded to or from servers uses internet bandwidth to push files along the network at various speeds. Every time you upload a file to your ISP, surf the web or use an audio application, you are using bandwidth.
Bandwidth is a bunch of wires or fibers connecting servers to a network. Depending on the grade of the wire it determines how much data is coming across the network where your web-site is hosted. When someone attempts to get more data than can be handled by the network, the whole network slows down.
ISPs can put a limitation on bandwidth at certain times during peak periods or charge you a flat fee per month for bandwidth usage. If you go over the flat fee, then they charge you extra for using extra bandwidth. Some ISPs will shut down the transmissions until traffic is more stable on the network.
You can load most files (images, sound files, videos and flash scripts and other programs) in your web site. This excludes, of course, banners and pop ups and specific documents and images which are supposed to be loaded from a central server.
Bandwidth bandits link to images and other files directly to some other server instead of putting them on their local server. There are various reasons as to why they do this but one reason is to get as much bandwidth as possible to show their links and images. So they steal images or audio files each time the site is initialized meaning that they steal the bandwidth.
Theres certain ways that you can stop the person from stealing your bandwidth allocation. If they have an email, I would contact them personally or go through Network Solutions and do an IP lookup which will give you information on the person and who the site is registered to. You can always get in touch with the company that hosts their site too.
Bandwidth can be expensive and the last thing that you want is someone stealing it.