Heh, as others have stated, a little bit of lingo is needed. But let me go over the basics.
First, for compatibility. Most ecigs are compatible throughout the whole range of product lines and most will fit together using a standard called a 510 connector. There are a few that have older style or proprietary connections, so make sure it has at least the 510 connector if you're buying something. The second type of connector is EGO threads, most devices are compatible with both. Also, the battery voltages vary a bit, but are usually between 3.2-4.2 volts with most device being in the 3.6 volt range.
Second, there are different sizes of batteries that determine the overall class of the item. those are as follows:
Cigalikes: batteries are as small as a regular cigarette and resemble the size and shape of a standard cigarette. These have the lowest battery capacity and are what you see in the grocery stores and in a lot of convenience stores. Blu is a popular cigalike brand. 510 sized connector is what you need to make sure it has at this size.
EGO: most of the complaints about the cigalikes, is the low battery capacity, so they came out with a larger battery. these are about the size of a medium sized cigar. This is the most popular sized battery and it opens up a wide variety of atomizers. most of these have EGO threads as well . Some have a variable voltage (Aka VV) option so you can output the voltage to change the vapor output, or to match the capabiltiy of certain atomizers.
MOD: a mod generally means their battery can be replaced. there are three types of mods:
Mechanical (mech): These are usually just a threaded pipe or box with a 510 connector at the end. a switch supplies the voltage straight to the connector at the end. usually these have no wires in them. The differences between them is how the switch locks, how they vary the size of the pipe to adapt to whatever size battery you have in them. These provide the maximum output from the battery which may cause them to short and explode if you are not knowledgeable about what battery you put in and what coils you are usuing.
VV/VW mods: VV (Variable voltage)/ VW (Variable Wattage) mods are tubes or boxes with some electronics at the end. These are the largest of the mods, and the electronics allows you to change the voltage or wattage at will. Usually the electronics limit the amount of amp output, so not all coil builds are possible with these. These usually offer more protection than mechanicals by having limits set on them, so are much safer than a mechanical mod.
Hybrid: These are simply an atomizer hooked straight into a tube and switch. they are a single piece unit and cannot be adapted to as wide of a variety of batteries and atomizers and batteries as the previous two. Hybrids are the smallest mods of the previous two.
That is a brief explanation, but should give you a starting reference as to what's out there. As far as making your own mod, I highly advise you get a pre-made one first to learn everything about voltage and wattage issues. Not knowing can be dangerous. The lithium batteries in Ecigs are powerful and should be respected. Remember, you are putting a potentially dangerous battery inches from your face, and without knowing what you're doing, they can "vent" violently. The batteries themselves are no more dangerous than the cell phone batteries people you use every day, but the difference is the device the battery is hooked up to. If you decide to make the device, make sure you learn what you're doing before attempting to make one.
