Iron Bars

Not to be confused with Iron Ingots.

Iron Bars were added in the 1.8 "Adventure" update. They are a type of block that seems to occur naturally in strongholds and in NPC Villages. They serve a similar purpose to fences.

The iron bars can be placed in much the same way as fences or a Glass Pane.

History
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. .

Trivia

 * A 2x2 area made of the Iron bars can make a hole big enough for the player to fit through, but not rain, water, etc. this can be used as an easy entrance for an underwater house, but not an exit. Glass Panes work similarly.

Possible Bug
The iron bars do not connect to sticky pistons like they do with the walls. ++ being one block of iron bars
 * |being a wall block / piston

instead of | |++++| |    you get   | | ++ | | this      | |++++| |     this      | | ++ | | | |++++| |              | | ++ | |

regular block             pistons

This makes it difficult to make castle gates that are functional and look right.