Expanding on what the other have stated: When the current is turned on and off through a diode, particularly when it shuts off because the polarity is reversed, there is a tiny delay before the diode responds. In that tiny delay it behaves more like a resistor than a diode. It happens extremely fast, often just a few nS pass before it starts or stops conducting. At low frequencies the delay is rarely important but as the frequency increases it becomes more of a problem. Unfortunately, in SMPS the frequency can be quite high and there can be very fast reversals of polarity so the loss in a normal diode causes heat to be generated and the circuit loses efficiency. A schottky doped diode has faster switching characteristics which mean it spends less time in its resistive state and therefore dissipates less heat. As a bonus, the forward voltage drop is also a little lower which means power loss (W = Vf x I) is also less and you get more output voltage. The drawback is they tend to have poorer leakage current and poorer temperature stability so they are not a replacement for normal diodes in all applications.
Brian.