The Purpose of a Ferrite Core

Ferrite is the name given to a highly resistive ceramic compound formed when ferric oxide reacts with a metallic oxide, such as that found in cobalt or zinc. Essentially, a ferrite core is a base around which the windings of transformers and other electrical devices are formed. Its purpose is to suppress electromagnetic radiation.
Electromagnetic Interference
If many electrical systems are placed in close proximity in a confined space, there is a danger that they will interfere with each other electromagnetically. This phenomenon, known as crosstalk, can cause systems to malfunction or even fail completely.
Inductor
A device known as an inductor can be constructed by wrapping a coil of wire around a ferrite core. Electrical current passing through the coil generates a magnetic field, which, when the current is increased or decreased, acts to oppose the change. Inductors are used to flatten current spikes and filter high frequency signals.
Transformer
A transformer similarly employs a ferrite core — wrapped with windings of copper wire — to step up or step down the output voltage relative to the input voltage. A transformer transfers electrical energy from one circuit to another by electromagnetic induction.