Firstly, Nmos is ON when Vgate = high, and Pmos is ON when Vgate = low. Also with the threshold voltage, and base on conducting condition, we can see that nMOS switch is "weak" 1, and pMOS is "weak" 0.
Secondly, For Nmos switch, source is typically tied to ground and is used to pull-down signals, and for pMOS switch,source is typically tied to Vdd, used to pull signal up. And note that the MOS transistor is a symmetric device, this means that the drain and the source terminal are interchangeable. For a conducting nMOS transistor, Vds > 0V; for the pMOS transistor Vds < 0V (or Vsd >0V). :smile:
Hope this help!
Duc.