How does the automail work? An automail must be a masterpiece, as the user seems to be able to use it as a fully working arm/leg/... It seems however, that it's no regular artificial limb.

What's so special about an automail? How does it work? Are there any drawbacks?
@kuwaly's answer covered most of it, but here's a bit more information as well.


  What's so special about an automail?


It's basically a prosthetic limb which has been optimized for fighting. Its name comes from auto- (as in "automotive") and -mail (related to armor). They were originally designed to give more freedom of motion to uses of prosthetics, but were soon fitted to combatants who had lost limbs.

There is nothing extraordinarily special about these limbs except their versatility and wide variety of uses. Additionally, this type of technology did not exist in the real world during the time Fullmetal Alchemist is set (early 1900s).


  How does it work?


There are dozens of little parts to it, but it is largely composed of motors, tubing, and the external metal covering. Here's a couple images of its components (click to enlarge):

 

It has no internal power source; it draws its power directly from the impulses of the nervous system to run the intricate motors and other mechanics inside it (and of course, to control which ones activate when). So, when the synapses are sent, each part contracts with its motors, giving free, versatile movement.


  Are there any drawbacks?


Of course&mdash;otherwise, why wouldn't people just cut off their limbs? ;)

kuwaly mentioned three major drawbacks:


It is painful to have installed (nerve connections are painful&mdash;in real life, too).
It is prone to breaking (just like a bone or limb).
It creates a reliance on an automail mechanic for tuneups and fixes, and mechanics can make mistakes!


There are two drawbacks that weren't mentioned, though. One is that it is quite painful during pressure changes; recall that when Ed went to the field with Pinako to dig in the rain (2009 anime), he was actually forced into vomiting due to the aching pain of the pressure system. The second is that the material can work against you; iron and steel can cause issues like frostbite when in cold areas, and are also quite heavy. (Some alloys alleviate this issue, though.)