ferrite rings
Hi wafula.
Ferrite rings can be used in many circuits, as inductors or chokes as there known, they are good for mains borne interference, often used in filters of all kinds, they are also excellent for RF interference on reception, couple of turns or more of coaxial cable and a lot of unwanted noise is gone, depends on the application as for inductors.
They are almost always used in conjunction with mains suppression capacitors and other components to make up a mains filter.
There also used in SCR circuits and many others, some are two lead components others are wound on a ferrite ring, some no ring just air for the core, commonly measured in micro H Milli H and Henry's, Dave.
http://en.wikipedia.org/wiki/Ferrite_core