You have seem them on the end of computer cables, these black blocks. Inside is a ferrite core and they help suppressing noise radiating from the cable. These ferrites present a high impedance to the RF interference thus canceling them out and preventing cables radiating this unwanted noise.
Using ferrites against EMI by EMACS
Excerpt from the document:
One of the most used yet least understood techniques for reducing both incoming and outgoing RF interference is the application of ferrite sleeves to cables and at interfaces. This tutorial is meant to shed some light on the use of ferrites, and also presents some comparative frequency domain measurements both to illustrate some of the points, and to give designers an idea of what they might expect in using and specifying a ferrite component.
These were made by Elmac Services with actual ferrite samples in a specially-designed test jig, using a spectrum analyser and tracking generator.
To continue reading see the pdf below: