ORIGINAL: Slo-V Flyer

Voltage by itself determines the RPM of the motor (ignoring a load being put on it like driving your truck on the ground, etc.). Assuming you're running the VXL 3500 Kv motor, the motor will rev 3500 RPM per volt you feed it. So at 11.1 volts, that's 11.1 x 3500 = 38,850 RPM.

So to answer your basic question as also stated above, battery voltage determines speed (or efficiency by letting the motor rev more but gearing down), and the mah (milli amp hours) rating is the capacity of the battery (and usually it also determines how well the battery can handle higher amp draw).

Going up in voltage however, also will under normal circumstances cause the motor to pull more amps (AMP is what the motor creates torque with, to put it simply) proportionally. Going from 2s lipo to 3s lipo will not only increase your rpm potential 50% faster, but possibly your amp draw and the resulting torque by 50%. This is assuming the motor CAN actually convert the amp it pulls into torque and doesn't max out and instead convert it to heat. And yes so in case you missed it, more voltage can lead to hotter temperatures since the resulting increase in amps (depending on motor efficiency and also on your gearing, and your setup) can cause more system heat.

The more amps your system draws, also the faster you drain the battery, or less runtime in other words. So.... you can either take that hit and go faster, OR you can gear down a few teeth in motor pinion size and go not as fast, but reduce amp draw, resulting in better heat, and run times, and still have a bit more power.

You don't want your motor to reach over 170-180*F (200*F and most motor magnets lose strength permanently!) and same for ESC, and for lipo you DON'T want to let the battery touch 140*F or you risk puffing and ruining the pack. 120*F is plenty warm for them.

These are mostly general rules of thumb type things. Excuse me for rambling if I'm repeating things you already know.

Hope this demystifies some things for you.