It's a DC to DC converter, which uses transformer to increase or decrease the DC voltage level on the load.
In this case, a step down transformer is used, so the output DC voltage is much lower than the input source voltage (it's based on the secondary winding of the transformer).
The switch (transistor) changes the pure DC voltage into Pulsating DC voltage; in order to be used by the transformer (either increasing or decreasing the level) then the output of the transformer is also a pulsating DC voltage, so a filter (capacitor) is used to minimize ripple factor, sharing a steady power to load and storing energy in order to supply the load whenever it needs (therefore, the load current and load voltage won't come to zero).
