Automail is a metal and mechanical prosthetic used in the Fullmetal Alchemist series.
In the world of the series Fullmetal Alchemist. If a person was to lose a limb there are option that don't exist in the real world. Real world prosthesis do not have the ability to continue fine motor functions. In this world the automail is connected to the nerve endings of the body. There are moving parts, gears, valves, and wiring that allows much finer range of motion. The ability of the motion is normally dictated to the training of the wearer and the skill of the Automail Engineer.
Since fusion of the mechanism requires the nervous system of the body be connected by some form of interface. This requiresa certain level of drastic surgery and is considered extremely painful. It's due to the great level of pain that not everyone opts to use automail in a place of a basic prosthetic.
The positive side is that automail can provide increased strength to the user.
Since automail is basically advanced mechanisms. They can also be enhanced with weapons such as knives, guns, and in some cases chain saws.
To be an automail engineer is to be well educated in the human nervous system as well as advanced mechanics in order to meld the two together in a working form.