A vaping 'Atomiser' consists of 3 main parts; a battery (usually rechargeable), a heating coil and a liquid resovoir. The basic principle is that pressing the 'vape' button causes a liguid (or 'juice') to be dripped onto a heating coil which in turn is heated by the battery. Heating coils can purchased with different strengths (coils are rated by their resistance in OHMS - usually 1.2 to 6.5 ohms) and batteries can be purchased with different voltages (usually 3 to 8 volts).
The resulting combination will give a power output in watts (similar to a domestic electric fire) and thus enoght temperature to vapourise the eliquid (juice). Generally, the ideal wattage is approximately 7 watts, lower wattages may not produce any vapour, higher wattages may cause the eliquid to burn. Buying a starter kit or complete atomiser means the manufacturer has done the required calculations and the atomiser will be set for the greatest efficiency.