I’ve seen a lot of confusion surrounding this subject, and not a lot of good explanations in terms of esk8. This is my go at explaining the terminology, calculations and physical assembly of lipo (lithium-polymer, aka LiPo, li-po or Li-Po) battery packs, and is designed to supplement the information already found the FAQ post. This all applies to lithium-ion cells as well, but the nominal voltages are slightly different. Let’s begin.
Lipo Cells
A single lipo cell has a nominal (which basically means “in name”, or the average) voltage of 3.7 volts. That can go as low as 3.2v or 3.3v when depleted or as high as 4.2v when fully charged. Lipo cell sizes vary greatly and can go from a tiny 40mAh (which stands for milliamp-hour, or the number of milliamps that can be supplied for 1 hour; it’s a measure of the energy contained in the cell) cells to huge 10000mAh cells (or even bigger). For esk8 you generally don’t want less than about 5000mAh because your range will be too short to really have fun. When you’re assembling batteries (which, incidentally, are called batteries because they’re a “battery” of cells) you need to determine what voltage you want to run at, which brings us to the first number you’ll encounter.
The “S” Number
If you want to run at 6S (generally accepted to be the minimum for esk8), that would be 3.7v (which, as you’ll remember, is the nominal voltage of lipo cells) multiplied by 6, which is 22.2 volts. If you went 10S that would make your battery 37 volts. If you had a 6S lipo, you could take the plastic off the outside (CAREFULLY! If you pierce a lipo cell it will catch fire or explode) and you would find 6 individual lipo cells connected in series. So that’s the “S” rating on batteries: it’s the number of cells connected in series. Connecting in series increases the voltage by total voltage of the newly-connected pack, so adding a 3S (11.1v) pack in series with a 6S pack (or two other 3S packs, which is 22.2v) would make the total voltage 33.3 volts.
The “P” Number
Now, the “P” rating on packs means very little. It’s how many batteries are connected in parallel. Connecting batteries in parallel increases the capacity (the mAh amount) of the total pack by the mAh of the newly-connected pack. So if you have a 6S 5000mAh pack and you connect another identical pack in parallel, the total battery capacity will be 10000mAh (and the voltage will still be 6S). Notice I said it’s the number of “batteries” connected in parallel, not “cells”. If you have a 10S2P pack, that’s two 10S batteries connected in parallel. So if you took the pack apart, you’d find a total of 20 individual cells. The reason the “P” number means very little is that it only tells you how the pack is physically assembled, not how big it is. If your cells are 2000mAh, that’s a much smaller 10S2P pack (it would be 4000mAh) than if the cells were 5000mAh (which would be 10000mAh).
The “C” Number
The “C” rating on batteries lets you calculate how many amps the battery is rated to discharge at any one point. It just stands for “capacity”, so a 20C battery is 20 times the capacity in amp-hours. That means a 2000mAh 20C battery can only discharge a paultry 40 amps, but a 5000mAh 20C battery can discharge a much heftier 100 amps. So yes, they do expect you to do the maths yourself. It’s worth noting that however you connect your batteries together, the “C” rating never changes. However, since C stands for capacity, if you connect two packs in parallel the capacity will increase, which means the amount of amps that can be discharged doubles. For example, if you have a 5000mAh 20C pack, that’s 100 amps of available discharge current. If you connect another identical pack in parallel it’ll make a 10000mAh 20C pack, but since the mAh has doubled, the available discharge current has increased to 200 amps. It should be noted, however, that these are theoretical values. In reality, while the battery may be fine supplying the theoretical amps (depending on how accurate the manufacturer’s C-rating is), the wires probably won’t. A lot of batteries have 10 or 12 gauge wires, which is not enough to carry 200 amps; the wires will literally melt 
For more information on wire gauges refer to @jmasta’s post for regular wire and @Namasaki’s post for silicon wire.
Calculating Range
And finally, the calculation you use to get the approximate range of your board is Wh (which stands for Watt-hours, or the number of Watts that can be supplied for 1 hour; it’s a measure of the energy contained in the pack) divided by 10 equals the approximate number of kilometres you can go. So a 500Wh pack (a really big pack!) should have an approximate range of around 50km (which is a hella long way to ride on a skateboard!). To calculate the Wh of your pack you simply multiply the number of Ah (amp-hours) by the voltage. So with a 5000mAh 10S pack, that would be 5Ah (because 5000mAh is 5Ah) times 37 volts, which is 185Wh, and should have about 18.5 kilometres range. Note that this calculation is intended for street boards (i.e. standard urethane wheels). The range will be significantly reduced if you’re on a Trampa/AT style board (i.e. mountainboards with pneumatic off-road wheels).
Something worth noting is that when assembling packs, you must always ensure all the batteries you intend to connect together are the same voltage, the same C-rating, the same mAh and same chemistry (lipo or lion). Otherwise you risk wearing out your battery much faster or simply destroying it instantly (which, of course, risks a lipo fire or explosion). Preferably you should simply always use the exact same batteries from the same manufacturer.
Please feel free to suggest changes or corrections 
Quick-reference summary:
- “S” is number of cells in series
- “P” is number of batteries in parallel
- “C” rating is the available discharge rate (eg. 3000mAh 20C = 60A discharge rate)
- “V” is voltage, which is determined by the number of cells in series (eg. 3S = 3 * 3.7 = 11.1V)
- “mAh” is milliamp-hours, or the number of milliamps that can be supplied for 1 hour
- “Ah” is amp-hours, or the number of amps that can be supplied for 1 hour
- “Wh” is watt-hours, or the number of watts that can be supplied for 1 hour
- Ah * V = Wh (eg. 5Ah * 22.2V = 111Wh)
- Two lipos connected in series only doubles voltage
- Two lipos connected in parallel only doubles mAh capacity
- “C” rating never changes, but increasing mAh increases available discharge rate
- Wh / 10 = approximate range in KM (eg. 111Wh / 10 = 11.1KM range) - only accurate for street boards
