Iron Bars

Iron Bars are an addition to the 1.8 "Adventure" update. They are a new type of block that seems to occur naturally in strongholds (revealed in the same image, posted by Jeb on his Google+ account on July 1st 2011) and in NPC Villages. They may serve a similar purpose to fences, or act like a gate or door.

On July 11th 2011, Notch revealed two new screenshots of a stronghold (in two different brightness settings) where the bars can be clearly seen alongside other common stronghold blocks. The lighting differences have been explained a bit (though not necessarily completely correct) by a user on the forums.

The iron bars can be placed in much the same way as fences or glass panes

Crafting
As of Beta 1.8, Iron Bars can be crafted. To craft Iron Bars, place a 3x2 grid of Iron Ingots in a crafting table. This yields 16 Iron Bars. Tutorial