Similarly for input high, NMOS is ON, PMOS is OFF. The output is shorted to ground via NMOS. Hence we say, "output is pulled down to ground", hence the name pull down transistor for NMOS. Its withrespect to the output voltage level that we use the phrases "pull up"/"pull down" read more
(2) If n1 corresponds to VDD (Supply Voltage) and n2 is in floating state, then if there is a transistor (X1) across n1 and n2 that is turned ON, then the voltage at n2 will be driven up to VDD. In this case, X1 is called pull-up transistor as it pulls up the voltage of n2. read more
The pull down device must be say an nmos transistor, while the pull up device can be a resistor an nmos load transistor or a pmos transistor. In case of CMOS inverter the pull down is a switching pmos transistor and the gate of the nmo and the pmos are tied together to form the input terminal of the inveter. read more
This condition means that their output is either grounded when LOW, or floating when HIGH, so an external pull-up resistor, (Rp) needs to be connected from the open-collector terminal of the pull-down transistor to the Vcc supply. read more