Do you ever wonder what that cylinder is doing attached to the cable of your USB or mobile phone charger? It usually has no label, no apparent use, and is a bit heavy. Usually it is near the end of the cable that goes into the mobile device.
It is called a ferrite bead, basically it's a ring, or rod with a hole (in topology this is the same, well a donut and a mug is somewhat the same) but I digress, its function is to soak up unwanted electromagnetic interference. Instead of twisting wires like in Cat-5 wrapping a segment of cable with iron can also soak up interference.