Q:

Where does Elune fit in the Warcraft cosmos?

What exactly is Elune?  Is she an Ancient, a Naaru, or something else?  I understand that the Ancients were on Azeroth before the arrival of the Titans.  However, where does Elune fit in in the grand scheme of things?  

A:

Not really known, but probably a naaru.
The answer posted by Auditore is a rough summary of the lore surrounding Elune before recent events.  She was a distant diety worshipped by the night elves, had little to no physical presence or shape beyond being a goddess of the moon, and granted them many blessings.  She's somewhat well liked in lore circles, if only for being probably the only diety that seems to exist in the game that doesn't seem to come down to Azeroth just to get killed off for one storyline or another.
The newest piece of lore that probably prompts this question is the storytelling surrounding Light's Heart, a fragment of a naaru that gets dumped onto Azeroth by outside forces.  For those confused, a naaru is basically a crystallized energy being of Light introduced in The Burning Crusade and used sparingly outside of Draenor lore.  The fragment is identified as a sentience core of a naaru long thought to be lost, and possibly led an army of light against the Legion on planets greatly distant.  Unfortunately, the piece of sentience within can only be unlocked by a descendant of that naaru (of which the only known one is inconveniently vaporized by the Legion as soon as you learn this).  Khagdar comes up with a fragment of lore that states Elune birthed the naaru, despite no other link ever being made between them.  Reasoning that a piece of an ancestor might work as well as a descendant, you take the Tear of Elune to unlock Light's Heart, and it works.
Presuming everything we're being told upfront is true, that pretty much outs Elune as a naaru, or something incredibly similar.  However, Blizzard seems to be downplaying this particular piece of lore and not making anything explicit, possibly because a lot of players preferred Elune the Mysterious Goddess to Elune the Crystal Angel Thingy.  So this is possibly as much of an answer as will ever be given.

A:

TL;DR: Unknown so far, but she's fairly up there in the cosmic hierarchy... in fact, she may be the source of what everyone calls "The Light".

There is very little in the games or novels about Elune, outside of vague quasi-religious information. That said, here's what we know about Elune:

She is called "The Mother of Azeroth" by the Night Elves. Let's stick a pin in that one...
She appears to be the oldest known deity in Azeroth's history
She is known & respected by all three of the pre-Titan races (Trolls, Tauren, Night Elves)
The various Earthen races seeded by the Titans also know of Elune from ancient times
To these races, Elune represents love, compassion, & nature
Khadgar claims in Legion that Elune gave birth to the Naaru
The moon of Azeroth is considered the physical representation of Elune
The Night Elves call Azeroth's moon "The White Lady"
The Tauren call Azeroth's moon "Mu'sha", and they believe that it and the Sun are the eyes of a deity called "The Earthmother"
Lastly, the Tauren teach that The Earthmother created Azeroth in the Age Before Memory... long before the Titans showed up

And, as a spoiler for those without Legion:

 We now know that Azeroth is essentially a giant "egg" or birthing matrix with a nascent (or "baby", if you prefer) Titan at its center.

Given all of this, we are left with two options. Elune is either

A female Titan, and literally the mother of Azeroth - possibly one located inside the moon.
She is something higher than a Titan, possibly even the cosmic source of "The Light" itself, and the moon is just her avatar.

Personally, I think it's the latter option - she's literally the cosmic embodiment of Light that seeded the universe with life. This would make her the mother of both the Titans AND the Naaru, as well as pretty much all naturally-occurring life in the universe. 

A:

Elune is the major deity worshipped by the night elves. She is associated with the larger of Azeroth's two moons, the White Lady, and is the mother of Cenarius. In tauren mythology, she is known as Mu'sha, and is the left eye of the Earthmother.
Elune is one of the few true deities in the world, and the most powerful Eternal. In the world's infancy, she protected all living things and allowed them the chance to grow and thrive. Whenever she found violence, she would cast her calming influence across the land so that peace and healing might be given another chance to thrive.
Thus, it was for many centuries, until arcane energy began to leak into the world through the Well of Eternity. Despite her attempts to guide them away, she watched in horror as her spiritual children among the kaldorei were drawn to the Well and seduced by its power. Though her companion Malorne and their child Cenarius joined her faithful among the kaldorei in an attempt to stave off the arrival of the Burning Legion, the demons spilled across the land in a wave of death and destruction that even the moon goddess could not prevent. With the Great Sundering and the terrible loss of life that came with it, the demons were defeated.
Just as she felt she had once again brought Azeroth toward a new era of peace, the human kingdoms fell under the corrupting influence of high elven magic, and crumbled into chaos. Shortly thereafter, the orcish Horde came to Azeroth and sparked a new era of warfare that threatens to consume the world like the War of the Ancients did so long ago. Though her kaldorei followers have returned from their time in the Emerald Dream, it has yet to be seen if the moon goddess will take any direct action during this time of trouble.
Elune never engages in combat of any kind, nor does she allow it to occur in her presence. Though she never physically manifests in the world of mortals, when she involves herself in a violent situation she arrives invisibly and undetected using her song of peace to calm the combatants until sunrise.

Source
P.S. The very first link I got when i googled "Elune Warcraft Lore". Just Saying :) .

