Iron Bars

Iron bars are blocks that serve a similar purpose to fences but are 1 block high instead of 1.5 blocks high.

Obtaining
Iron bars can be mined using any pickaxe. If mined without a pickaxe, it will drop nothing.

Natural generation
Iron bars can be found in strongholds, villages, and igloos if they generate with a basement.

Usage
Iron bars can be used to create arrow slits when placed between a door and a solid block. This creates a small gap between the door and the iron bars which mobs can see through (and will attempt to attack the player through), and which arrows can be shot out of.

Torches cannot be placed on the top of iron bars while blocks can.

Endermen seen through iron bars will not become angry.

Arrows cannot be shot through iron bars. If shot between the gap, it will look like the arrow is floating in the gap.

Placement
Iron bars can be placed in much the same way as fences or glass panes. Before 15w31a, when placed without anything on its sides, they will appear in the shape of a from the top, but when something is placed next to it, it can also become flat,  shaped, or  shaped, depending on the blocks surrounding it. Now, after 15w31a, when placed without anything next to it, iron bars appear as 2x2 pixel bars.

History
On July 11, 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 correctly) by a user on the forums.

Trivia

 * A 2x2 area made of the iron bars can make a square shaped hole big enough for the player and most mobs to fit through, but not rain, water, etc. that can be used as an easy entrance for an underwater house, and even an exit if pistons are used. Glass panes work in the same way.
 * If a sign or a painting is placed on the iron bars, it appears to float in the air.
 * Iron bars also prevent fire from spreading, i.e. surrounding a fire with iron bars will prevent wooden items from being lit aflame.
 * Despite apparently being thin enough to do so, iron bars do not let a ghast's explosion touch the block beneath them as long as the block is protected from other angles of the explosion affecting it. This, in theory, means you should be able to use iron bars and sticky pistons to make openable/closable shutters.
 * If an iron bar is connected to a block and a rail is placed on the block, the top texture of the iron bar disappears.
 * While curing a Zombie Villager, iron bars placed nearby will speed up the healing process. The same happens with beds, and both these things occur in the stereotypical prison cell, encouraging players to imprison them.
 * If placed on top of a single cactus, only the corners can damage.
 * The player cannot shoot arrows through iron bars.
 * Since iron bars are 1 block high, the player can normally jump over iron bars unlike fences.
 * In the code, iron bars are known as fenceIron.