# MOSFET switching on

I have a doubt. Consider an N-MOSFET: which is the voltage that can switch on it? The voltage between Gate and? Sometimes I read "between Gate and Bulk", sometimes "between Gate and Source", sometimes the ambiguous sentence "Gate voltage".

Then I have another question, related to the previous one. Consider the following pass transistor circuit:

How can the signal at the Gate activate the MOSFET? It must be high with respect to what point (moreover in this circuit Drain and Source are not fixed points)?