I found this on the DIY ELECTRIC CAR WIKI:

Skier Analogy:

Imagine your electrical system as a mountain with chair lifts and some skiers on it.

The voltage of an electrical system is comparable to the height of the chairlifts, i.e. the amount of potential energy per unit (skier or electrons). If you have ten small chair lifts (12V) you can either put them all in a row up the hill which adds the height of each getting your skiers pretty high up the hill (giving 120V) or can have them all next to each other allowing more skiers to go up the mountain but not as high (12V).

Amps are the number of skiers travelling on our little mountain circuit (from the bottom up the chair lift then back down the slope to the bottom). In the example above stacking the chairlifts up the mountain (series connection) gives a lot of height (10x voltage) but limits the amount of skiers (1x amps) that can go up the mountain. Having the chair lifts all next to each other (parallel connection) allows lots of skiers (10x amps) to go through the circuit but they won't go as high (1x voltage).

You can think of power sources (like batteries) as chair lifts i.e. they add energy to the system by taking the skiers up the hill. Power consumers (motors, resistors etc) are like the downward slope of the hill, the energy that was given to the skier by the chairlift is used when they go down the runs on the slope. The cables that join everything is sort of like the skier cutting across the mountain without going down very much. The skier can get to the slope (eg. motor) where he wants to go without losing much of his height (cables have a small resistance but generally don't drop much voltage across them).

Power is like an instantaneous (not influenced by time) measure of how much fun people are having in your resort. You get enjoyment happening when people travel down the runs on your hill. Having a few people (small amps) on very long runs (high volts) is the same amount of overall fun (watts) as having lots of people (high amps) on a small run (low volts). When you have too many people (amps) on the same small run they start to melt the snow away (melt wires, burn out motor etc.) but if your skiers have too much height and speed (volts) they might break the more fragile bits of the circuits as the go past them (i.e. motor brushes).

Water analogy:

Volts are the speed of the water moving down the pipe. 5mph? 10mph? Those are both speeds. They do not specify "how much" water, just "how fast."

Amps are a little trickier to explain but think of it as a width of a pipe at any given moment. Amperage changes based on load, but in my analogy, it's like a pipe expanding to get the amount of water needed through. Amps also don't specify "how much" water, but "how wide"

So, you have "how fast" (volts) and "how wide" (amps). What's that give you if you multiply them? "how much" (watts). A watt is how much power is being used at a given moment. It does not, however, say how much power is used in total. It's just at a given moment. If you want to measure the "Reservoir" of water, you need watt-hours. Those are like gallons of water.

So, I hope this clears up some basic electricity physics.

On to series and parallel wiring applied to batteries.

Batteries in series add voltage but don't change amperage.

Batteries in parallel add amperage but don't change voltage.

That means:

2 6v 10ah batteries in series make for 12v 10ah.

2 6v 10ah batteries in parallel make for 6v 20ah.

It's actually the same amount of watt-hours (multiple volts times amp-hours for watt-hours).

Formulas:

Watts = Amps * Volts

Amps = Watts / Volts

Volts = Watts / Amps

Watt-Hours = Volts * Amp-Hours

Batteries in series = add voltages

Batteries in parallel = add amp-hours

Motors in series = divide voltage by # of motors

Motors in parallel = divide amps by # of motors

Source:

http://www.diyelectriccar.com/forums/showthread.php?t=6535