And how it relates to your e-Cig:
The battery in the video is your e-Cig (I use an eGo-C Twist).
The resistor in the video is the resistor built into your atomizer. You often see these as anywhere from 1.5ohm - 2.8ohm.
The lightbulb in the video is the coil of your atomizer, the part that heats your e-juice. I'm using the Vision v2 clearomizer.
Volts = Ohms x Amps
In simple terms, Amps (short for ampere) measure how much current is flowing out of your battery. An Amp-hour measures how much current a battery can provide over a length of time. mAh is milliAmp-hours, 1/1000ths of an Amp-hour. So, the more Amps you're pushing out of your setup, the more drain you're putting on your battery. You can figure this out by taking the Voltage you have your battery set to divided by the resistance of your atomizer.
So let's say I have 2 clearomizers which are 1.8ohm and 2.8ohm. Below is a chart for appropriate voltages for these devices.
The Safe Vaping Power Chart
For those of you with more advanced gear with higher voltages the chart below will help you find that sweet spot. I find I am referencing it constantly. Happy vaping!