These types of kits are the ones you see blowing massive amounts of clouds, but that's not all Sub-Ohm/DTL (Direct to Lung) kits do.  What makes these kits different is that the coils that are used are below 1.0ohm in resistance and instead of vaping the kit like a MTL (Mouth to Lung) kit you inhale the vapour straight into your lungs. 

These kits tend to vape at a higher power/wattage than the other types of kits, so a different type of e-Liquid is used, 70vg 30pg is the norm, these are the bigger 50ml/100ml bottles you typically see in shops. This type of e-liquid is used to provide a smoother vape at a higher wattage. 

We wouldn't recommend anyone go above 6mg of nicotine in your e-liquid if using one of these types of kit, due to the vapour production being higher.  That said this type of kit is perfect for those who want to blow clouds and have intense flavour. 

These types of kits are normally composed of two sections, the Sub-ohm tank and the Mod:

The tank is made up of the glass that holds the e-liquid, the coil that vapourises the e-liquid  and the chamber that connects the whole tank together. This is the part of the kit that most impacts the flavour and cloud production. 

The Mod is the part of the kit that supplies power to the coil in order for it to work. On most mods you can vary the wattage/power among other settings. If your kit needs external batteries to work this is where you would put them.