Here is the explanation by ecf.
'Mods' are high-performance models, of the second and third-generation class. The basic definition of a 'mod' is that it must be able to use replaceable, generic batteries. Almost all e-cigs of all types use 3.7 volt batteries. Any device that can have the batteries taken out and replaced is a mod. There is only one exception to this rule (the Darwin, which currently has a non user replaceable battery).
Although a mod was originally just a device with bigger batteries, nowadays they often have extra features. The advantage is they use large, generic batteries; and instead of using the small cells found in cigarette clone models (known as minis or first-generation models), the larger batteries allow better performance and can be bought at lower cost from many sources online. The standard features of any mod therefore include bigger batteries that last longer and are cheaper per unit of power. A mod that is built to be small and compact, and about the same size as a mini e-cigarette, will lose out on battery life and power - because where batteries are concerned, size is everything.
Mods are made by people/hobbyists, they grab something, modifying it into a PV.
