Ferrite rings are toroidal (doughnut) shaped, or cylindrical shaped pieces of ferrite (a ceramic compound).
Ferrite rings help to protect against Electromagnetic Interference) (EMI) and Radio Frequency interference (RFI). They prevent interference moving to a device or from a device
Ferrite rings can suppress noise without grounding. Ferrites are able to attract magnetic flux. When a cable is passed through the ring, the magnetic fields created by the cable are concentrated within the ferrite ring. This magnetic energy is converted to heat and the ferrite dissipates this by magnetic loss.
Ferrite rings can often be seen with cables threaded through them. They are common place in electronics to help with noise suppression. The cable can also be wrapped about the ferrite ring numerous times. Ferrite rings can be incorporated into broadband transformers, pulse transformers and common mode chokes.