AC (Alternating Current) - The polarity is continuously reversing or 'Alternating'. In the case of a UK mains electricity supply this happens 50 times every second. This is known as the 'Frequency' and is quoted as 50 Hertz. The rating plate of an appliance will normally show something like:

200 - 230 Volts AC    50 Hz

With an AC supply, there is no Positive (+) and Negative (-) as clearly they repeatedly change places. However, we do have Live and Neutral. This is tied up with the earthing arrangements for an AC supply
DC - (Direct Current) - The polarity is constant and must usually be observed. Most electronic equipment requires a DC supply which usually comes from either batteries or the mains via a power supply unit. 

DIODE  (also known as a rectifier)


Put a diode in the circuit and current
can still flow but only in one direction.
Reverse the battery and light stays
off because the diode will not let
any current pass

Flame Conductance and Flame Rectification

Flame conductance relies on the fact that an electric current will pass through a flame e.g. the flame is a 'conductor' of electricity. 
This system can be 'fooled' if the electrode touches earth as it appears to the electronic control as a flame.

Page D-6 of the Viper book
Flame Rectification relies on the fact that although a flame will conduct electricity, it does so far better in one direction than the other. In other words, the flame acts like a diode. This makes for a more robust system because if the electrode gets bent and touches earth, the electronic control can tell the difference.
Page D-7 of the Viper book.