Here is the briefest answer to the question about the difference between MTL and DTL. You can understand the meaning directly from the picture above in 1 second.
MTL – Mouth To Lung Vaping
Mouth to Lung (aka MTL or M2L) is the vaping style that most closely replicates smoking. Most newcomers to vaping that are switching from smoking will start by MtL vaping as it feels the most natural way for smokers to inhale.
Features: Weak vaping feel. Newbie’s choice.
DTL – Direct To Lung Vaping
Direct to Lung (aka DTL, Direct Lung Hit, Straight to Lung, or D2L) is the vaping style that is most commonly used by experienced vapers. For transitioning smokers, it feels like a completely alien way to inhale at first. However, it is essentially the same thing we do when breathing normally.
Features: Strong vaping feel. Old hand’s choice.