A manaforge is a nether-collecting structure that harvests mana from the Twisting Nether. Manaforges suck energy from Nether (same energy that mages use at their spells) and it goes directly along the pipes to Tempest Keep, into Kael´thas use. Now ethereals smuggle energy in manacubes to the Isle of Quel´Danas, where Kael´thas and his army has moved.
Their original purpose is unclear, but a questline with Exarch Orelis in the Inn at Area 52 reveals that Kael constructed the Manaforges using technology taken from Tempest Keep. Thus, they appear to be of Naaru design, but only because the core technology is. Perhaps they used physical materials from Tempest Keep, or perhaps Naaru design is so integral that to use the technology necessitates using the very same physical forms, materials, and sigils.
According to Exarch Orelis, Manaforges are a form of energy recharger, which was never designed to be used so close to land. This use is ripping Netherstorm apart and accelerating its deterioration.
Interestingly, the pipeline connected to Tempest Keep actually doesn't run into the nearby ground, as it does with the Manaforges. Instead it terminates far away from it, near Eco-Dome Midrealm. How the pipes connect is unknown.
The blood elves control all manaforges except Manaforge Ultris, which was taken from the blood elves by the Voidwalkers, along with the mine beneath it.
The manaforge that has been destroyed: