THANK YOU!!!!! LOL I totally get that; thank you for the light switch metaphor, and for the aside about RF interference, etc. This was exactly what I needed. You made my day!
(And thank you to everyone else for your responses, as well! They were very informative, but Osgeld hit the nail on the head in terms of what was perplexing me. I kept thinking the input pin was being connected to a conventional circuit, so it didn't make sense that you'd be connecting it to + and - in rapid succession, because, well, what's the other end connected to?? But now I realize that it's an isolated "sensor", almost, that registers whether it's being exposed to a + or a - charge...
Osgeld:
your over complicating it (though it can be a complicated subject)inputs can be thought of like a light switch, you turn it off or on, but you are not part of the loop (well you could be but that would be bad), instead of a switch the inputs are connected to transistors, which take very little current to change states (because they are very high impedance)
connect a input to +5 and the transistor switches on, connect it to ground and it switches off (it may be backwards I forget how they are arranged in the chip, doesnt really matter for this)
Since the transistor takes very little current to reach one state or the other due to its input impedance, if you left it unconnected it would wildly fluctuate as it picks up electrical energy from the air (RF interference, body capacitance whatever ... it actually makes an ok random number) so you need to "steer" (yea I dont like that term either) the pin to one state or another by using a pullup or pull down resistor
edit: and no you will find this type of setup on many digital electronics throughout the ages
edit: I bolded the key line -- that one doesn't connect the pin to +5V or Ground because you're making it part of a conventional circuit, but in order to expose it to a change in electrical potential that will trigger a HIGH or LOW reading... right?
edit 2: and then I expanded on my "Thanks to everybody else" aside...