"Pull-Ups and Pull-Down Registers"

I think you mean Pull-Up and Pull-Down Resistors [1]:

If you connect a resistor from a point in your circuit to the positive supply line, it will tend to "Pull" the voltage at that point "Up" towards the supply voltage; hence such a resistor is called a Pull-Up (or just "pullup") Resistor;

Similarly, if you connect a resistor from a point in your circuit to the 0V line, it will tend to "Pull" the voltage at that point "Down" towards 0V; hence such a resistor is called a Pull-Down Resistor.

You will also hear of "Active Pullups" - this is where an Active device (transistor, FET, etc) is used in place of the simple, passive, resistor. The advantage of this is that the pullup can be controlled by a suitable signal at the gate/base.

You will also hear of "Weak" and "Strong" pullups: this reflects how "hard" the pullup can "pull" - ie, how much current it can deliver.
A low-value resistor provides a "Strong" pullup.

The 8051 Port pullups are nicely illustrated in the "80C51 family hardware description" document from Philips:
http://www.semiconductors.philips.com/acrobat/various/80C51_FAM_HARDWARE_1.pdf

While you're there, you ahould also download and read the other 80C51_FAM_*.pdf documents:
http://www.semiconductors.philips.com/products/all_other.html


[1] The pullups may well be controlled by settings in a register!