Pull-ups vs Pull-downs

As the name suggests, pulling anything up seems way better than the not-so-encouraging downward tug :)...
But seriously speaking, actually as serious as I can get, I always tend to forget all that I studied in college (from the math to the digital circuits...). But the most basic concept I come across everyday is a Pull-up resistor (register as per the Koreans' :D, I mean they tend to pronounce it like that, and its kinda taken its toll over me toooo :)) ). Well, statistically speaking, verbally I come across a pull-down resistor, but it turns out, in the real world, its the pull-up counterpart which scores...

Pull-down resistors : Used to hold the input to a zero(low) value when no other component is driving the input.

Pull-up resistors : Used to ensure that the inputs to a logical system settle at expected logical levels if external devices are disconnected. They are also used at the interface between to logical systems (which might be different from each other) which might be operating at different supply voltages.

In TTL logical systems, the un-connected inputs are inherently left floating, and as such requiring a much lower pull-down resistance value to force the input low. But this results in higher current consumption. For that reason, CHIN-UPs :D are preferred when you encounter a TTL circuit :)...