What happen when digital pin is set as input or output?

I have just bought Arduino Uno and have a question.
When we set digital pin as output, the voltage differences between that pin and GND pin will be 5v or 0v to light the LED or turn off LED. But what happen in digital pin when it is set as input. I mean that voltage dropped on it. It will be 5v, 0v or something else? cause I am countering problem when learn about pull up and pull down resistor.
Thanks in advance.

When an Input, there is very little current flow into the 328P - like a uA (0.000001A).
So the input voltage will be whatever is applied.
If you use the internal pullup resistor ( pinMode (pinX, INPUT_PULLUP ); ) an internal 20K to 50K resistor is connected. There is not internal pulldown resistor.
If you use external resistors, then 10K is generally a good value for pullup or pulldown to source or sink about 0.5mA of current.