Thanks swedishfish! I couldn't remember where I had posted my response, and didn't want to retype it
E-Cig 101, an e-cig basically has three components: 1) battery = power; 2) atomizer = vaporiser; and 3) juice delivery = cartridges, tanks, juice bottles (juice fed mods).
All e-cigs and mods are combinations of these components, and the mods are designed to optimize them, which they do to one extent or the other.
Batteries deliver the power that drives the atomizer. Generally, if you give the same atomizer higher voltage, the vape gets hotter and delivers more throat hit. The larger the battery, the longer it lasts, and the bigger the mod needs to be.
Atomizers heat up and turn the juice into vapor. Generally, the lower the resistance (measured on ohms), the hotter the vape.
The battery voltage and the atomizer resistance have to work together to get a good vape. That is why you will see people talking about a 3.7v battery and a 1.8ohm atomizer, or using a high resistance atomizer with a 6v set up. Most people find a battery that gives them the size or longevity that they prefer and an atomizer that works well with that battery.
For those who are still searching, or are using batteries that have a gradual power loss while being used, there are variable voltage mods, which either manually, or automatically adjust the voltage being delivered to the atomizer.
Finally, the juice delivery. I use a REO Mini, which is a "bottom fed" or "juice box" mod. The pv itself has a bottle of juice contained inside so there is no refilling needed during the day. The provari, and others like it, are used primarily for dripping or use add-ons (like MAP tanks, etc.) to provide the juice to the device.
Hope that helps.