There are many coils available on the market, all with different ohms ratings and you do need to be sure that you choose the correct one for the tank you use always check the packaging. The Series 2 coils are 1.5ohms and are compatible with the Aqualites Series 2 tank.
A heating element or coil is a looped resistance heating an alloy wire, a single coil has one alloy wire which is heated and a dual, you guessed it…. has two. The power or ohms is split between the two coils which in turn heats your e-liquid to a point of vaporization. The main difference is that it provides a smoother vape as the heat is transferred a lot quicker. It also provides more vape plume which some people like but it has a knock on effect that it does use more battery power so keep your charger handy or upgrade to at least a 900 battery!