Mouth to lung (MTL) style tanks have a slightly restricted airflow and can be used as you would have puffed on a cigarette. Usually intended for coils above 1 ohm.
Not huge cloud producers, but flavour is more pronounced. Most are designed to be used with standard liquids with a low-medium VG percentage.
There is some crossover, and some tanks are both MTL and DL capable and each vaper has their own preferences and vaping style.
+ P&P if applicable