Techmarine
Techmarines are Space Marines who have been trained by the Adeptus Mechanicus in the ways of the Machine God. The senior Techmarine in a Chapter is generally known as the Master of the Forge.
Techmarines often start as Marines with an aptitude for machinery; if they attract the notice of their Chapter leadership, they may be sent to Mars for a number of years to train with the tech-priests. After their training is complete, their armour is modified to fit their new servo-arms and backpack and is usually painted red to show allegiance to the Mechanicus - leaving only the pauldrons to display the Chapter colors and badge. This sets them apart from their battle-brothers and means they are not often completely trusted, though to technophilic Chapters like the Iron Hands it is a badge of honour.
When they return to their Chapter, Techmarines join their Chapter's Armoury and maintain the Chapter's vehicles and weapons. In some Chapters like the Salamanders all Marines know the basic maintenance procedures for their equipment, leaving the Techmarines free to work on more exotic weapons and war machines.
Among the Space Wolves, Techmarines are called Iron Priests, and will noticeably not paint their armour red, leaving it grey. The Iron Hands have two types of Techmarines. The first are Iron Fathers, who oversee the more "spiritual" aspect of the machine. The second rank is Ironwrought, who are full Techmarines.
Warpsmiths are their evil counterparts among the Chaos Space Marines.
Dawn of War II
Relic decided to use the the Techmarine in Dawn of War II as one of the three commanders for the Space Marines. The techmarine is the least aggressive the marine commanders, he's built as a ranged unit (while the Force Commander and Apothecary are extended to lead assaults, he's expected to stand back shoot things, and does inflict more damage shooting than they do), and focuses on improving ranged damage of friendly units, has abilities that support vehicles, and can bring emplacements that reinforce squads without needing to go back the base, along with bringing in sentry turrets. His style generally focuses on getting to a position and holding. He's trickier to use than the Force Commander and Apothecary, but his abilities can quite deadly on certain maps, especially ones where he can place his turrets in strategically important locations that maximize what their firing arcs can hit while minimizing the directions the enemy can come at them.