Warpsmiths (not to be confused with Warsmiths) are basically Chaos Techmarines, introduced in 6th edition. Many of them are loosely allied with the Dark Mechanicus. Unlike Techmarines (who revere technology), Warpsmiths attempt to subjugate it. They are also masters of fusing the daemonic and the physical, which, in layman's terms, means they inject Chaos into all their warmachines. Warpsmiths tend to be obsessive and believe that mankind's destiny is limited by his mortal nature, and thus they see combining the power of Daemons with the mechanical and organic as the answer to circumvent this problem.
Because of this, all Warpsmiths are engaged in an eternal quest to combine the strengths of all three of these elements -- mortal, daemon and machine -- whilst eradicating their weaknesses. They would conquer the galaxy and remould it into one giant, tainted flesh-engine if they could. The Warpsmiths themselves seek to embody this unholy fusion of man, machine and daemon; in their search for perfection, they often become more metal than flesh. Some Warpsmiths are little more than a brain and a spinal cord wired into a metallic approximation of a Chaos Space Marine; pincer limbs, Mechadendrites and fusion claws sprout from the Warpsmith's altered form next to Melta crucibles and searing welder blades.
Unlike Warsmiths (who, per Iron Warriors tradition, replace mutations with bionics), Warpsmiths have bionics from being actual Techmarines.