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Pull-Up and Pull-Down Resistors

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These circuits can be built with or without the switches.

Remember a switch is like a resistor. When it's on, the resistance is very close to zero. When it's off, the resistance is very close to infinity.

Pull-up and Pull-down Resistors

In these circuits

the pull-down resistor pulls the voltage down to zero. If the pull-up switch is pressed, it pulls the voltage up to whatever the + supply is.
the pull-up resistor pulls the voltage up to whatever the + supply is. If the pull-down switch is pressed, it pulls the voltage down to zero.

These circuits are used to reset 555 monostable circuits and counters. Microcontrollers and desktop computers usually have a reset switch. These will all use one of these circuits.