Answer: Ferrite beads
This chunky, hard-plastic cylinder at the end of many cables found in your home and office (USB cable, video cable, etc.) has a specific function. In the hard plastic housing is a piece of ferrite-infused ceramic that is designed to suppress high-frequency noise. Essentially, the ferrite bead acts as a passive low-pass filter, blocking both the energy from "outside" sources that could affect the function of the cable and absorbing the energy that could emit the cable and interfere with nearby equipment. The largest part of the energy is absorbed by the caterpillar and then released as a tiny amount of heat into the ambient air.
Why is it important to suppress high-frequency noise? The noise cancellation serves two primary purposes. On the one hand, it prevents the electrical disturbances that are generated in one device from disturbing other components. For example, interference from a computer does not affect the performance of the monitor. Second, it prevents electrical energy and radio energy from leaking and disturbing surrounding equipment such as televisions and radios. Ferrite beads are one of the simplest (and certainly one of the cheapest) types of interference filters that can be installed on existing cables when needed, as you can buy inexpensive snap-on models that attach directly over existing cables.
Courtesy of B & H.