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). 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 (or possibly chains or vines) 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.