Top left circuit is AND.
Bottom left, NAND.
Top right... Or.
Bottom right is of course NOR.
These Resistor/Transistor Logic Gates are probably simpler in design than you imagined huh? Each of them can be built as you see them here into a real circuit, and they're really just as easy as this! Have a couple transistors handy? Pop them in a breadboard and see for yourself!..
