E Cigarette MOD stands for "Modification" to a standard personal vaporiser. They come in various shapes and sizes offering various power options and have no internal wiring resulting in a more robust unit. They provide the experienced vaper with the ability to vape longer between recharges. Additionally this extra power vapourises the e liquid in a fraction of the time of conventional e go battery devices.
Most newbies to the vaping world however will start with an e go starter kit or disposible cigarette before upgrading to a MOD. Whilst both types perform the same function, in that they vapourise e liquid using a heated coil, the MOD can accommodate a much wider range of Tanks.