Mods are the power source for your device. They come in two major categories. Series or parallel.
A series circuit is a circuit in which resistors are arranged in a chain, so the current has only one path to take.
A parallel circuit has two or more paths for current to flow through. Voltage is the same across each component of the parallel circuit. The sum of the currents through each path is equal to the total current that flows from the source.
Mods also come regulated or unregulated.
A regulated mod is a device that contains a circuit board to regulate the current. Any device that has a variable voltage or wattage setting is a regulated device.
An Unregulated mod is a device that does not contain a circuit board for regulating power flow through the device. Mechanical mods, Series or Parallel box mods (without displays or variable voltage or wattage).