The naaru are a dimension-travelling race of sentient energy beings with a deep affinity for the Holy Light of Creation that empowers the paladins of Azeroth. They are bent on stopping the Burning Legion. The naaru blessed the draenei with this power in an effort to help them in their struggle.
The name is pronounced by holding the A sound for slightly longer, approximately "NAH-roo".
When Sargeras and the Burning Legion came to the world of Argus and began corrupting the eredar race, Velen, an eredar leader, prayed for help. The naaru appeared, offering to take Velen and any other like-minded eredar to safety. Velen and his faction accepted and began to call themselves draenei ("Exiled Ones"). The draenei hastily escaped Argus aboard the Dimensional Ship provided for them by the naaru.
During the draenei's journey, the enigmatic naaru race blessed them with Light-given knowledge and power. The naaru explained that there were other forces in the cosmos that would stand against the Burning Legion. One day the naaru would forge them into a single unstoppable army of the Light. Deeply affected by the naaru's words, the draenei vowed to honor the Light and uphold the naaru's altruistic ideals.
Due to the interests of the Legion, and later certain Azerothians, in Outland, the naaru moved Tempest Keep to Outland for a more direct observation. The Keep was stormed by the Blood Elves, but the Sin'dorei found only a single naaru within the ship.
It appears that the naaru have advanced knowledge of magic and access to otherworldly materials and reagents that have not been seen in Azeroth. Tempest Keep looks much sleeker than most buildings seen on Azeroth; however the texture slightly resembles that of "Wizard" architecture. The only factual knowledge we have is that Tempest Keep has "automated" defenses such as the ability to fly, and teleport itself and anything in it.
It can be overwhelming to the lore considering that these features are easily associated with science fiction movies, but if one looks at it logically it is not so technological after all. The automated defenses are most likely more powerful counterparts of the Arcane Towers built by the Kirin Tor. The necropoli (such as Naxxramas) that the Scourge employ can also fly and move vast distances, so that ability can also be obtained through magic.
In short, the goblins or the gnomes are more advanced than the naaru in terms of sheer technology. What the naaru use appear to be actual machines which are almost as advanced as that of the gnomes or goblins. But these are greatly enhanced by deep and powerful magic to the point where they give the illusion of being highly-advanced sci-fi technology. For a contrast, the Cuotl  in Rise of Nations: Rise of Legends are the reverse: their technology is so advanced it is indistinguishable from magic. It's the same sort of starships we're used to, but powered by arcane and Light magic instead of nuclear fusion or dilithium.
Naaru appear to be made of shards of light rotating around a central axis.
It is unknown whether or not the naaru have separate genders. K'ure, A'dal, and M'uru have all been described using the pronoun "he." It is entirely plausible that all three of these beings happen to be male, but given their nature as energy beings, it's possible that the naaru are genderless.
From the quest line in Oshu'Gun involving K'ure, we discover more about the naaru life cycle. When a naaru dies, its energies become depleted and transforms into a void. The void is a space of pure shadow in which it consumes all spirits and souls around it and transforms them into voidwalkers as elemental off-products.  The great amount of void energies at Auchindoun are created by the corpse of the great naaru D'ore who was interred by the Draenei at Auchindoun. However, after a period of time, or with a proper source of holy magic, a naaru corpse can regenerate its lost energies of the light and return to full power. Thus, the naaru life cycle is one of light, darkness, and resurrection. 
Factions and locations
There are various naaru factions in-game, some of which you can earn reputation with.
There is also a abducted naaru within Silvermoon named M'uru, who is held captive by channeling blood elf sorcerers. This naaru is the source behind the Blood Knights (blood elf paladins) who seek light from the naaru.
At [Kael'thas Sunstrider's] signal the elves stormed the dimensional fortress, quickly defeating its automated defenses and claiming its satellite structures. The only real threat the elves discovered was a lone naaru who had stayed behind to maintain the keep's defenses. With some difficulty, Kael'thas subdued the energy being and sent it to Quel'Thalas so that the magic-addicted blood elves could feed upon it. Back in the capital city of Silvermoon, Magister Astalor Bloodsworn was not content with this idea. After long months of study and experimentation, he and his fellow wizards learned how to manipulate and corrupt the naaru's luminous energies. In the end, the wizards devised a process by which the powers of the Light could be transferred to recipients who had not earned such abilities. Instead of feeding upon the naaru's magic, the blood elves would wield the naaru's Light-given powers themselves.
|A'dal||Master of Shattrath City||Alive||Holy Template:Class Icon Priest Small|
|D'ore||Buried in Auchindoun||Dead?||Holy Template:Class Icon Priest Small|
|G'eras||Sha'tar Vendor in Shattrath City||Alive||Holy Template:Class Icon Priest Small|
|K'ure||Led the draenei exodus from Argus||Alive||Holy Template:Class Icon Priest Small|
|Mi'xi||Drove away the attackers of Shattrath||Alive||Holy Template:Class Icon Priest Small|
|M'uru||Technician of Tempest Keep, power used to create the Blood Knights||Alive||Holy Template:Class Icon Priest Small|
|O'ros||Protector of the Exodar||Alive||Holy Template:Class Icon Priest Small|
|V'eru||Naaru in Shattrath||Alive||Holy Template:Class Icon Priest Small|
|Xi'ri||Leader of the Sha'tari forces against Illidan Stormrage||Alive||Holy Template:Class Icon Priest Small|
- Speculation on the origins of the naaru suggest that perhaps they were allies of Sargeras when he was still fighting evil. Perhaps Sargeras recruited these beings to aid him in his struggle, but later betrayed them when he was corrupted by evil.
- There has been speculation that they are somehow related to the (non-titan) gods of Azeroth, such as Elune. The Warcraft Encyclopedia confirms that while Elune is a true goddess, neither the titans nor the naaru are gods. This does not preclude a connection with Elune, just that She is not one of them.
- With the revelation that Khadgar and the Expedition have been fighting the Legion alongside the naaru, it is highly probable that the Alliance (and perhaps even the Horde) is the precursor of the "army of the Light" the naaru promised the draenei would be a part of.
- The naaru act in a similar manner to the angels of earth mythology and religion. See Myth for more information.