Just a quick note on APV types, to add to Kent's info above.
1. A 'mechanical APV' has no wiring, and takes a generic Li-ion cell (or cells). Examples are the Reo (boxmod), Xhaler (tubemod), tubemods with a bottom end button, and hybrid RTA tubemods with a bottom end button. The circuit is made by a mechanical button, not a switch, and there is no wiring of any kind. These are the simplest devices of all, and therefore the most reliable - some have a lifetime guarantee as a result. All they generally need is an occasional clean of the contacts. They will last longer than any other device as there is essentially very little to wear out in normal use, and they can easily survive an incident such as total immersion. In fact you could clean one by washing it out.
2. An 'electrical APV' or 'basic electrical APV' has a wired electrical switch in the positive line, and takes a generic Li-ion cell. Examples are the Silver Bullet (tubemod) or any plastic case 3.7v boxmod. They are simple devices with just a short positive wire to a switch, and a wire from the switch to the positive pole on the connector. They are easy to fix because there is only a switch or soldering to repair/replace.
3. An 'electronic APV' is one with a circuit board, and either takes a generic Li-ion cell or has a built-in Li-Poly cell. Typically they are 5 volt regulated devices or VV devices. Examples: 5v boxmods, Provari tubemod, Darwin. These devices offer by far the most features, but cost more and may have reduced reliability. They may need to be treated carefully.
These are three three basic classes of APV. Sometimes people get the first two mixed up. If the device has a wired switch, like an SB, it isn't a 'mechanical' APV.
There are also sub-classes like hybrid mechanical-electronic APVs - a 'mechtronic APV' perhaps - which is what you get if you put a Kick circuit board in a mechanical APV like an Xhaler. That maybe has some advantages, because if you wreck the electronics by liquid incursion then you can at least throw away the board, fit a spacer (like out of an Xtar charger) and vape on...