[Gordonjcp]

Internally? Imagine a grid of wires where each address is one vertical wire and each bit of the output is one horizontal wire. The output wires have resistors pulling them up to 5V. To select an entry in the ROM you pull its corresponding vertical wire to ground.

Now here's the clever bit - there are diodes between the vertical and horizontal wires. When you pull a vertical wire to ground it pulls all the horizontal wires connected through diodes to ground, putting a 0 on those pins.

In a real "mask programmmed" ROM the grid is more square, so there are a lot of horizontal lines grouped in 8 bit bytes.

In an EPROM the diode is replaced by a little MOSFET. By applying a high voltage to its gate it'll stay charged basically forever, switching on and forming a "0" in the output.