MTL or Mouth to Lung is a popular vaping style specific to those who prefer flavour over cloud production and/or for those who vape liquids with higher nicotine strengths.
MTL tanks restrict the airflow entering the chamber that houses your coils, thereby providing the user with a tighter draw. This tight draw can more closely mimic the draw of a cigarette and as a sensation, is highly sought after by those looking for that cigarette style vape and is best suited to flavour chasers and new comers to vaping.
MTL tanks also use a higher resistance coil. The advantage of a higher resistance coil is that it requires less power to operate. Not only does this mean that you get great, consistent flavour, you also get longer battery life which should help you get through the day on a single battery/charge.