A ferrite core is a type of magnetic core on which the windings of electric transformers and other wound components such as inductors are formed. It is used for its properties of high magnetic permeability coupled with low electrical conductivity (which helps prevent eddy currents).
There are two broad applications for ferrite cores which differ in size and frequency of operation: signal transformers are of small size and higher frequencies, power transformers are of large size and lower frequencies. Cores can also be classified by shape: there are toroidal cores, shell cores, cylindrical cores, and so on.
The ferrite cores used for power transformers are working in the range of low frequencies (1 to 200 kHz usually) and are quite big in size, can be toroidal, or shell or C shape and are useful in all kinds of switching electronic devices (especially power supplies from 1 watt to 1000 watts maximum, since more powerful applications are usually out of range of ferritic single core and required grain oriented laminations cores).