Pull up and pull down resistors are used on the inputs of logic gates. Logic gates have a very high input impedance and their input current is very low; this allows the designer to use pull up or pull down resistors to force the input to be at a desired logic level by connecting the other end of the resistor to the 5 volt source for logic 1 or to ground for logic zero. The designer can be assured that the resistor will have almost no voltage drop, because of the very low input current of the logic gate.
Pull up and pull down resistors are used on inputs to logic gates where the input(s) are not "driven" to a well defined logic level by being connect to the output of another logic gate. Because these inputs are not connected to anything, the designer uses a pull up resistor to assure that the input is at logic 1 or a pull down resistor to assure that the input is at logic 0. If the designer does not use the pull up or pull down resistor, then the unconnected logic input may vary its logic state due to noise and other parasitic coupling of the adjacent electronics. Again, the pull up resistor is a tool that assures that an otherwise unused (disconnected) input is at logic one and the pull down resistor is a similar tool that assures that an otherwise unused (disconnected) input is at logic 0.
Let's use the example where you have a 3 input AND gate but you only need to AND two signals A and B. You use a pull up resistor on the third (unused) input to assure that it is at logic level 1; this allows you to use the gate, because A?B?1 = A?B
A similar example for a pull down resistor is a 3 input OR gate. You use a pull down resistor on the unused input, because A+B+0 = A+B.
Pull up or pull down resistors should, also, be used on gates that are are not used at all; this done to assure minimum power consumption. To be sure that you have designed for minimum power consumption , the inputs should be pulled up or pulled down is such a way as to make the output of the gate be logic 1. Therefore, the inputs to all unused AND ano NOR gates should be pulled up, and the inputs to all unused NAND and OR gates should be pulled down. For unused XOR gates 1 input should be pulled up and the rest pulled down. For unused XNOR gates pull all inputs down.
There is one special case for a pull up resistor used with an open collector logic output. Open collector gates are used when there is a large load at the output and the resistor must be designed to drive the load to logic 1.
Pull up/down resistors are used at inputs because many types of logic circuits have such high input resistances that their input voltage would be indeterminate if there wasn't a resistor there to pull it up/down.
They also reduce the risk of damage to the circuit from static charge- a static charge on the small capacitance of the gate input could otherwise lead to a dangerously high voltage.
I understand that it keeps the reading from an input at a specific logic level but how does it do the pulling in the first place???? How does it even change anything? I thought that resistors are only something to add extra resistance, hence the name. Ofcourse, it probably has something to do with the way the current flows, but that's where I'm stuck. It's obvious that I'm missing some very important assumption that would allow me to understand this, so please, explain the way you think about it and understand it. I've been through my book's sucky explanation and 10 other websites' similar explanation with nothing, so maybe I need some unconventional way of explaining this now very annoying topic.
