Direct lung style tanks have a more open airflow and are mostly designed for sub-ohm vaping for large cloud production. These tanks work best when you inhale directly into your lungs. Sometimes they are also called sub-ohm tanks or subtanks.
Most are designed to be used with thicker liquids with a higher VG percentage and at higher power settings. Subtanks tend to consume more liquid, and users normally opt for a much lower nicotine level.
Using a true DL tank with a mouth-to-lung style can result in a harsh taste and could encourage flooding and leakage.
There is some crossover, and some tanks are both MTL and DL capable and each vaper has their own preferences and vaping style.