A Mod is the battery holding device that usually includes some type of “fire button” that is used to begin sending energy to the heat element of the atomizer. Some mods come with a battery embedded within it and some have a tray where the user can insert a battery. Some mods have a smart logic board in them that allows the user to configure how much energy is sent along with a lot of other useful things, but some are very simple devices with a mechanical button that just makes contact with the battery.