The Adeptus Mechanicus is the chief scientific and industrial organization within the Imperium, responsible for virtually all of its manufacturing and research needs. It is also a religious organization that follows the Cult of the Omnissiah, also known as the Machine God, the source of all knowledge. Despite the official state religion of the Imperium being the Cult of the Emperor, the Mechanicus has a special exemption written by the Emperor himself. But to keep relations between the Mechanicus and Ecclesiarchy smooth, they nominally identify the Omnissiah in the Emperor. In addition, they have their own forge worlds, which are ruled directly by the Mechanicus and provide most of the Imperium's technology. In exchange, the Mechanicus receives any archeotech found within the Imperium for study, and gains access to Astropaths and Navigators from Terra.
The religion of the Adeptus Mechanicus grew out of a need to placate the beings known as Machine Spirits, remnants of fully-functional artificial intelligence that went rogue after the Age of Strife. Tech-Priests will perform various bizarre rituals on even the simplest machines in order to coax them into cooperation, as an angered Machine Spirit can be quite dangerous. The Mechanicus is also obsessed with reclaiming lost fragments of STC technology, as innovation and independent discovery is generally frowned upon, or else impossible due to the amount of knowledge lost over the millenia.
In battle, most low-level priests will be lent out as Enginseers to the Imperial Guard to keep their vehicles maintained. By contrast, Space Marines have battle brothers trained by the Mechanicus themselves called Techmarines. As their own fighting force, the Mechanicus primarily use the Skitarii tech-guard and semi-autonomous robots of the Legio Cybernetica, as well as the Imperial Knight Houses loyal directly to the Mechanicus as opposed to the wider Imperium. They are hoarders of technology, possessing a few war machines unique to themselves. They are also responsible for maintaining and operating the Titans, massive land walkers and the heaviest weapons available to the Imperium, and the Ordinatus weapons, massive and uniquely-made superweapons used only against the toughest foes when all other alternatives have failed.