MTL / DTL

MTL means Mouth-To-Lung
DTL means Direct-To-Lung
There are MTL vapers and DTL vapers. It depends on the ohms of the coil and the type of airflow and mouthpiece the ecig has.

With MTL you inhale the vapour, so it's in your mouth, then take a second inhale (also know as taking it back) so it goes to your lungs.
With DTL you just take one inhale and it goes straight to your lungs.