The household electricity our North American appliances use is delivered at 120VAC (volts alternating current) 60 Hz (60 cycles/second). When the electric current flows it rises from 0 volts to 120 volts back to 0 volts and then it falls to negative 120 volts and rises back to 0 volts. This rise and fall completes one cycle (1 Hz) and is called a "sine wave." To convert 240VAC to 120VAC, for example, a transformer reduces the height of the 240 volt sine wave in half, creating a 120 volt sine wave that can be safely used by all types of 120VAC appliances, while a solid state converter chops the sine wave at its positive and negative 240 volt peak, reducing the power in half but creating a chopped waveform that can ONLY be used by NON-ELECTRONIC heating appliances.
NOTE: Transformers and solid state converters do not convert cycles (60 Hz North America vs. 50 Hz overseas).