PullUp and PullDown Network in CMOS
Pullups pull the voltage up towards more positive values, and pulldowns pull the voltage down towards more negative values.

PEMOSFETs have a negative threshold to turn on, so putting them in the pullup role means that they will conduct when their gate voltage drops, and vice versa for NEMOSFETs. Putting them both in series in the same circuit and tying the gates together results in a natural inverter.