The buck converter is a solution for DC; it uses an inductor to store some of the energy in the incoming DC (in the inductor's magnetic field); once it is 'charged', a switch is thrown which cuts off the incoming DC, and in a circuit including the inductor and the load, the collapsing field creates a current which runs around that circuit, using electrons present in the conductors.
They are often found in switching power supplies.
The basic actual circuit has a switch, and the inductor, in series in one line from the input to the load (the other is generally ground); there is also a diode across the two lines, after the switch.
The switch is closed, and current starts to flow; through the inductor, and through the load. When the voltage across the load is slightly above the desired output voltage, the switch is opened.
The field around the inductor starts to collapse, producing current which flows through the load, and the diode - which is needed to complete a circuit for the newly-'activated' electrons to flow through. (That connection has to be a diode, to prevent current flowing through that path when the switch is closed.)
When the output voltage has dipped slightly below the desired level, the switch is then closed again, and the cycle repeats.