Minecraft wiki entry for Iron_Bars, associated text: The first image of the bars that Jeb posted, where the iron bars can be seen to the right of the center.
Image posted by Notch, here we can see iron bars to the left and right of the center.
Naturally occurring iron bars at a blacksmith's shop in a village.
A jail cell made out of iron bars and cobblestone.
Water does not flow through the holes in the bars, but the player can fit inside them.
Natural-generated iron bars found in the End surrounding an end crystal.