Mouth-to-lung (MtL) is essentially the same as how most people drag on a cigarette. You suck the vape into your mouth and then inhale into your throat or lungs.
MtL users typically run their devices at a minimum of 5 watts, with some going up to as high as 14 watts!
DtL vapers fill their atomisers with lower nicotine strength e-liquid, typically around the 0-20mg mark.