Techpriest
Techpriests are the tech support and religious engineers of the Imperium in Warhammer 40,000. They are united by a belief in Machine Spirits and the Omnissiah, and zealously guard what secrets of technology the Imperium has managed to scrape together as the Adeptus Mechanicus - the technology cult of Mars. Their core beliefs are that all technology is sacred, and everything that can be learned has already been done so - this inevitably leads to the Tech-Priests unwilling to either reverse-engineer other technology or develop new ones. Given that the Imperium has been at war for 10,000 years, this means that they've been doing fuck-all to help the war effort other than make more flashlights.
Tech-Priests are often found outside the Adeptus Mechanicus working as tech-support, fixing machines when they break with hour-long maintenance rituals involving sacred unguents, incense and litanies for the machine spirit to start working again. This is surprisingly effective, given that most rituals involve anointing the machine with sacred lubricants, cleaning and purifying components, and striking the mystical 'OFF' and 'ON' runes in that order. Rogue Trader ships sometimes carry Tech-Priests, not just for their invaluable technological aid but to also ferry them to a pilgrimage or holy place. They may also be found working for Inquisitors as emissaries from the Adeptus Mechanicus, giving aid or claiming artefacts in their name. A Space Marine who shows talent with the sacred rituals of technology may be inducted into and trained by the Adeptus Mechanicus, eventually returning to their chapter further augmented as a Techmarine.
Hereteks
Outside the Imperium are Tech-Priests who sided with Chaos and became Hereteks and other disciples of the Dark Mechanicus - corrupted engineers who forge horrific weapons of war for their masters. They are notable in that, being free from the stifling embargoes on research laid down by the Adeptus Mechanicus, they often create new technologies and discover new principles their loyal counterparts may have passed over as heretical. Given that they work with Chaos, often it was a good idea to ignore it in the first place.