The difference can sometimes escape most of us, and it's hard to find a device that does in fact use multiple batteries in parallel, but it's crucial to understand the big differences between the two battery configurations.
Let's get to the basics: Series means that there are two or more batteries 'stacked', so the voltage that do/are able to be put out is added, but the actual capacity stays the same. Parallel means that batteries are configured 'side-to-side', so the battery capacity is added together, and the maximum voltage stays the same. Again I learn best through pictures, so below are the battery configurations and how they effect voltage, and capacity. (Ah is amp hour, a larger term for capacity as compared to mAh - milliamp hour)
Now let's say you have a device that uses two batteries, it can either be configured to have more power output, or more battery life. It's how you see a device that can put out 200w, and with a regulated device you have the choice of not using all of the battery power at once (Like how I used an RX200 at less than 80w for the whole time I used it)
Sometimes you have devices that use both parallel and series, they do this by configuring the batteries in a '2 series in parallel' layout, essentially getting both doubled battery life AND doubled power output. Of course, the result is an outrageously large device that in no way fits in a pocket. (I'm looking at you RX300)
And here is a picture to explain:
Here's why this is important to understand: There is such a thing as an unregulated series device, the big difference is it acts like a mechanical mod (straight through voltage dictated by the coils resistance). This is a big deal because let's say you've built coils to work on mechanical mods before, you have to throw all that inherent knowledge out the window, because with a series device like that the output voltage is doubled.
Let's do the math really quick: a simple 0.25Ohm build on a mech at a full charge.
At full charge it puts out 4.2 volts, 4.2V at 0.25Ohm = 16.8Amps and ~70Watts.
Now you put that build on something like a Noisy Cricket (a two battery series unregulated device) and the voltage doubles, but the current threshold stays the same.
At a full charge it will put out 8.4V at 0.25Ohm = 33.6A and ~282 Watts
See why this is important now? You throw your regular old build on an unregulated device like that and not only will you be stressing the ever-living hell out your batteries, but it will be like vaping fire if you aren't expecting it.
Now, on the flip side of this, parallel.
It's hard to come across a device that has multiple batteries in parallel, I can think of maybe 3 off the top of my head. The reason for this being you can really only get 100W out of 1 battery comfortably, and even that's a stretch. But there are some people (like myself) that get everything we want out of vaping by using a comparatively measly 80 watts. But just think, if you can get a day and some out of a 3000mAh battery, imagine double or even triple that battery life. A man can only dream of an all parallel RX200S (I'm looking at you Wismec, make it happen).
Another thing to consider is that with more power available, the longer you can run the batteries out. If you have a 3 battery device that can put out 200 - 250 watts, but you're only using 70 - 80 watts, the device only has to put out maybe 3 - 3.5 Volts, meaning that you can run all those batteries to nearly totally dead (Usually an 18650 battery is 'out of juice' at about 3.0V) and get some extra time out of them before having to charge again.
In conclusion, if you're sticking with regulated devices, you may not know or even care what the configuration is, but it is helpful to know what you're getting into. Also to not naively think that just because there are 3 batteries doesn't mean it's going to last much longer than if you had 1. Now if you are using unregulated, and you have your eye on that sweet and sleek looking Noisy Cricket, this understanding is not optional, you need to understand that it is a completely different game playing with series unregulated, and not having the knowledge to know that a regular old build will not work the same, can have potentially dangerous consequences. Vape safe everyone!
P.S. Wismec; I'll be waiting on that triple parallel mod in my mail.