These come in two types. Powdered Iron or Ferrites. Both introduce permeability, that means an increased inductance for a given number of turns. Toroids look exactly like doughnuts and come in various diameters, thicknesses, permeabilities and types depending upon the requency range of interest. High inductance for the physical space occupied.
Various permeabilities are available.
Exceptional Q values when wound correctly and optimum core and windings selected. Wide range of diameters and thicknesses. Relatively low cost 7. Often simple to mount or secure mechanically. Nearly impossible to introduce variable tuning of the inductance.