What does battery life have to do with tablet inputs? Tablets (at least, the typical kind and not the kind with built-in screens like the Wacom Cintiq) use relatively little power (much less than, say, charging your phone via USB).

Different tablets use different technology. The Wacom tablets use an inductive charge (from the tablet itself) to charge a capacitor on the pen. This means that yes, you need a special stylus. However, because of it, you can get much more precise location and pressure readings. You also don't need to change any batteries in the stylus, as it's powered "over the air."