The Phyrexians were followers of a warped and twisted, but godlike in power, individual named Yawgmoth (who the Phyrexians did, indeed, deify after his death). They were fanatical believers in Yawgmoth's philosophy that mortal flesh was a fatal flaw, and pursuing immortality through mechanical enhancement was the higher calling of the human race. Later, other races—either willingly or unwillingly—had this vision shown to them (for example, the Chiss). Because of their reliance on, and in fact status as, machines, which the Yuuzhan Vong hated, the Vong declared a holy war on the Phyrexians shortly after their arrival in the galaxy, and were in fact instrumental in the eventual defeat of the Phyrexian invasion force. More metal than flesh, with often quite functional mechanical enhancements (for example, one of Phyrexian general Thrang's arms was a high-powered flamethrower), the Phyrexians were nightmarish in appearance, and inspired quite a bit of fear in their enemies. When they were wounded, they leaked oil instead of blood.
The Phyrexians were extra-dimensional invaders, not originally from this galaxy. They first appeared in the Unknown Regions, then after destroying the Empire of the Hand went on to invade Galactic Alliance space. They devastated remote worlds such as Endor, Zuni, and Taanab before the tide of battle finally started to turn against them at Bothawui. They were completely destroyed in simultaneous battles at Thyferra and The Valley of the Jedi.