I've gotten contradicting information on the subject. Some say that digital input pins send out positive voltage, and if it goes to ground (for example) it reads LOW but if the pin is connected to power, it will read HIGH. On the other hand, some say that the input pin is receiving voltage from either power (+5V) or ground (0V). which one is it?
Digital pins sink or source current. They don't provide negative voltages. If the pin is set to
INPUT then it is high impedance and will not "send out positive voltage".
However if you activate the internal pull-ups, as Ignacio said, then effectively the pin is connected to +5V via (approximately) a 50k resistor.