Volts: How much the electricity want's to move from one side of the battery to the other (through the coil along the way). But that doesn't tell you much by itself. Because you don't know how many electrons are moving.
We'll use the water analogy.
Volts are like water pressure. If you have high water pressure, you get more water per second than lower pressure for a given size pipe. BUT....you can't tell how much WORK you could do with water at a given pressure unless you know the size of the pipe. A small garden hose at high pressure will do LESS work than a fire hose at lower pressure.
So Watts are total WORK than can be done. That's why light bulbs are in watts...it tells you how much "work" is done. Electric motors and electronic gadgets too. And also your electric bill is in 1000 watts per hour (kWh).
So upping the voltage will push more electrons through which ups the watts too. Also upping the watts ups the voltage. It's all related.
I'm not sure that's your real question though. What I think you want to know is....."Why do I care about VV versus VW?"
VW - variable watts - Adjusts the voltage automatically based on the resistance of the coil that you screw onto it. It calculates it for you. To do this, it has to read the ohms of the coil, and VW devices usually have a readout that will tell you the ohms of the coil.
VV is just a "hotter or colder" dial.
If given a choice, VW is nice because it USUALLY includes bonus stuff...like a display and readout of ohms/volts/battery level. VV can just be a dial.