For this not to evolve into a boring lesson, I'll skip the history of ecigs (which you can find from Wikipedia).
The basics :
There are several options for one to choose from when it comes to ecigs. The main thing is that you need an power source, something that vaporizes the e-liquid and naturally the e-liquid itself.
In the beginning we had ecigs that most were done in three parts : Battery (power source), atomizer (heating unit) and cartridge(liquid storage). And even today it's still almost the same, only difference being that there's a lot devices with combined heating unit and liquid storage in one.
There are power sources that work when you take a drag out of them and those that require you to press a button. The latter is the one that dominates the market, even though automatic batteries are still sold and manufactured. Most tobacco look-a-likes are automatic batteries so that the feel, size and the way you use it resembles regular tobacco the most.
Threading (Compatibility) :
Currently at the market the two dominating threads are 510 and eGo. This said you can use almost any 510 threaded device on top of an eGo threaded battery (eGo/Tornado/iMist). Only difference on that would be the Super Tank units (that comes to mind), even though they are equipped with 510-threading their atomizer won't fit on top of an eGo due to it's flange design*. Also parts like 306 dripping atomizers use the 510-threading so they are compatible with those two (510 and eGo).
Most vendors do list the threading of their devices which makes it easier for you to look up spare parts (knowing what you already have).
There is still different threadings out there : 901, 801, 808, RN4081 (and all sorts of letter combinations to go with those).
So you can use any* 510 (and 306) part with eGo battery due to the compatible threading they both have.
There really isn't much limitation to what e-liquid you can vape with any device out there. Do-Note : Some VG 'heavy' liquids can be harder to use on cartomisers that have that have cotton type filling (the liquid won't move as well due to it's thick texture). And true, there are some devices that are especially designed for certain base ingredient (Like Bud PG & VG tank atomizers).
PG is the part that brings out more flavor and less vapor, when VG is the part that gives less flavor and more vapor (Pure PG vs VG comparison video from YouTube).
Lately a lot of e-liquid makers have switched into combination of those two ingredients (80PG/20VG, 70PG/30VG, 50PG/50VG). These e-liquids have a nice balance of flavor and vapor production, no matter what gear you are using to vape.
Understanding ohms (resistance) :
Smaller the ohms, less volts you need for it to produce nice vapor. Higher the ohms, more volts you need for it to produce nice vapor.
This meaning that a regular 510 / eGo battery (or a mod that has no built in boost chip) which work at ~3.7V will give nice vapor up to (around and this is just my opinion) 2.4ohms max.
ADVANCED : It's worth mentioning that it's not recommended to use dual coil parts with small batteries (510) as they will shorten the lifespan of the battery (due to the strain it causes).
Maintenance and Lifespan :
Cartomisers, clearomisers and atomisers are semi-disposable. Their lifespan depends on multiple factors : Liquid used, the amount vaped, ohms (lower will have shorter lifespan) and voltage used (with VV/VW devices you can give more voltage which will naturally shorter the lifespan the same way as lower ohms). Lifespan on those can be anywhere from days to weeks, average being around 1-2weeks. So how do you know when it's time to change? There won't be as much vapor and the flavor diminishes closer to the end.
Average battery can be charged again and again around 200-300 times, some even up-to 500 times. So over time they too will lose the capacity to hold it's charge. To extend the lifespan of your batteries and achieve the max use is to keep it clean. Best way to do this is cleaning the threading part before charging with a paper tissue or a piece of cloth, preferably at least once a day.
So where to begin?
If you haven't tried ecigs yet, you can get an disposable electronic cigarette. Just take the other end to your lips and inhale / exhale the vapor. When the liquid and/or the battery runs out, just dispose it at your local battery disposal.
Next step is to get an starter kit. These kit's have everything you need except the e-liquid (in most cases). So there is an battery (or two), charger for the battery(s) and something to house/vaporize the e-liquid. Some kit's can also have some sort of carry case that you can store your ecig in. One of our biggest sellers has been the New Users Starter Bundle that even has e-liquid with it (Filling tutorial, Chargins tutorial).
In general your best buddy when it comes to vaping is your Vendor and Youtube (we have our own channel with guide videos too). Just enter the name of an item you are interested into Youtube's search and you'll get reviews, how-to's and even videos just going around the item you are looking for.