The Aeternalae are hard to describe as beings, because they defy many natural laws. Beings of Chaos and Order, they exist in the plane of Infinity, the Third Realm and its subrealms of Makai and Reikai. The single most accurate description is that they're spirits representing the human condition. When you think of a mouth, you're not likely to think of it as two folds of flesh that peel back to expose part of the skeleton and the muscles behind it. You're likely to think of it as a mouth -- some essential meaning behind it, such as an object that communicates. When you see a person's eyes, you're not likely to think of it as a lump of occular tissue, you're likely to think of it as a part of a person's soul or basic essence.
This essence in combination with the physical sense of being is what makes us human.
Imagine, if you will, this essence taking on its own form. Existing in the boundless Ether and even our own thoughts. Free of physicality, but also intimately related to it. An evolution -- or perhaps a mutation -- of humanity wrought of its own state of mind. You can see this essence purified in paintings or expressions of humanity, one popular form being Japanese animations ("Anime"), which is famous for emphasizing the eyes due to the role the eyes have in communication or state of thought. This is the Aeternalae -- being born of this essence in ten different Continuums. In each Continuum, there is a person or people who created the Aeternalae. All ten creation stories (and their alternate timelines, if you can wrap your head around it) come together in the Third Realm to form the Aeternalae as a whole.
Being as Aeternalae are more the platonic ideal of humanity than real humanity, when they do manifest on a First Realm, they exhibit have some very peculiar 'physical' traits to them when they do . Their hair is always silken soft, and never needs the touch of a brush or comb. Their only body hair is eyelashes, eyebrows, and head. They are completely hairless everywhere else. Their skin is always flawless, and of a uniform complexion. While within the Influence, they have no need of sleep or food, though they commonly indulge in these things for enjoyment. They commonly say that within the Influence, they are able to “Live on love”.
Sometimes, child-like Aeternalae are created either through rather the mysterious pregnancies of the Athernalen or the conversion of children into Aeternalae. These young Aeternalae are called Lovetears.
Other species of Aeternalae exist, due to the wildly varied nature of the Third Realm, and the ability of the Third Realm that enables one's self to be defined as one wishes, though many of these species were created well after the Alphariginae arose, and tend to have much smaller populations, and thus receive much less attention. An example of one of these smaller Aeternal species is the Hanako.