Simple NOR gate (transistor-level) diagram
A goes to both the P and N transistors.  If A is low, then the top transistor conducts and the second transistor connected to A does not.  If B is also low, then its upper transistor conducts but its lower one doesn't.  The upper transistors pull C up, and the lower transistors do nothing.

Now, let A be high.  A's upper transistor doesn't conduct, but the bottom one does and pulls C down.

The series structure at the top means that both A and B have to be low in order for C to be high.  If either one is high, then C isn't pulled up and one of the lower transistors will pull it down.