The Adeptus Mechanicus (often abreviated to AdMech) is a organization in the Imperium of Man responsible for science, technology, engineering and most industrial production.
A trained and capable member of the Adeptus Mechanicus is known as a Techpriest. For basic production means, the Adeptus Mechanicus makes heavy use of cyborgs known as Servitors. Servitors are human beings who have had a portion of their brain replaced with cybernetic systems, making them effectively living robots. Servitors are created from both vat grown clones and convicted criminals. Most Techpriests themselves undergo a large number of cybernetic enhancements, including the installation of Cybernetic Limbs, Man/Machine interfaces, built in sensor systems, and (for those Techpriests who are quite likely to come into contact with hostile forces) weapons and targeting systems.
The central administration the Adeptus Mechanicus is located on Mars, and it's leader is the Fabricator General of Mars, who is a member of the High Lords of Terra. Individual forge worlds are also led by Fabricator Generals.
Despite being part of the Imperium of Man, the Adeptus Mechanicus enjoys considerable autonomy, as the Imperium would simply be unable to function without it. It rules over tens of thousands of forgeworlds (extensively industrialized planets sometimes completely covered in warehouses and factories), which possess most of the Imperium's industry, as well as commanding significant armed forces, including its own army, known as the Skitarii (or Tech Guard), and its own fleet of starships. They also control the Imperium's Titan Legions.
Despite it's role in Imperial society, the Adeptus Mechanicus is a highly ritualistic and religious organization. There is a religious attitude toward activities such as experimentation, research, development, repair, and maintenance. Lower-level members of the Adeptus Mechanicus simply perform repairs and manufacturing procedures by rote, with little knowledge of how the mechanisms they maintain actually work. They believe that each machine has it's own spirit that needs to be appeased to keep it functioning properly -- a belief that is usually, although not always, pure superstition. Among their more notable beliefs is extreme intolerance of artificial intelligence and technology of non-Human design and manufacture. Higher-ranking techpriests generally have a more scientific outlook on things and follow the rules more loosely.
Alongside the Imperial Cult, the Adeptus Mechanicus worships a deity known as the Machine God or Omnissiah, which some suspect to be in fact a C'tan known as the Void Dragon. This fact has led to some tensions between the Ecclesiarchy and the Adeptus Mechanicus.
The Imperium has lost a considerable amount of its technological capacity over the millenia. As such, the Adeptus Mechanicus spends considerable effort looking for ancient pieces of technology in hopes of regaining lost knowledge from the heyday of mankind. This search is done by specialised Techpriest Explorators. The most important of such relics are examples of Standard Template Construct technology.