LOL! It's mod as in "modified".
So creative folks went into their garage and went to work creating new e-cigs, better ways to use existing e-cigs, better battery delivery, better juice delivery... and some of the ideas are so great that they are now being produced and sold on different e-cigarette vendors' websites. Others were so easy that you can do it yourself (perfect example: youtube e-cigarette cart mods).
Many are recognizable as mods because they certainly don't look like cigarettes anymore (ie: flashlight mod, box mod, eclipse mint tin mod - yes, named for the parts they used to make the mod).