Kith is a sub-category of a particular seeming, representing more specific affinities such as the various elements or animals.
Each Seeming has it's own kiths, which are smaller subsets representative of the variations found within a seeming. It is important to note that a kith is not a social category, but only shows archetype of a seeming. That isn't to say some people might have prejudice against some kiths or seemings more than others, but they will still find it hard to strictly categorize the endless variety of the Lost.
Sometimes a Lost has unfortunately (or perhaps fortunately) only spent enough time in Arcadia to gain a seeming but not the refining features that create a kith and the blessings that come with it.
If a Kithless character later wants to gain a Kith, he must face the trials of the Hedge and return again into the human world, and the trial is no simple matter. A changeling must face themselves in the hedge as they walk into their own psyche and challenge themselves, either relying on skills they know nothing of, or having their specializations brought to the upmost limit.
Likewise, a changeling with a Kith can wander into the heart of the hedge and find the very personification of their psyche that makes them a member of that kith and tear it away (at the permanent cost of a willpower).
The following comes directly from the White Wolf Wikia:
Without any kith, Beasts are an indefinably wild and bestial sort. They possess clear animal features that are undeniable but not easily connected with specific creatures. They often have light fur, elongated and sharper teeth, ears or eyes just larger than humans’; these vary, since Beasts can have broader, flatter teeth as herbivores do, smaller eyes as some creatures do, the lightest feathers rather than fur. Whatever form of body covering Beasts possess is typically indistinct, so light and consistent that it’s impossible to tell between fur, feathers and very soft, flexible scales. Beasts’ teeth border just between an herbivore’s and a carnivore’s (similar to a human’s, but oddly… off) but are more pronounced in both regards. They are animal, but which?
Darklings are among the more abstracted seemings, notable for their bonds to the larger concepts of darkness, silence and night. Some of the Darkling kithless are hard to distinguish from Darklings who have a kith that manifests itself subtly. If anything, kithless Darklings are notable precisely for being even less notable, more subtle. They are fragments of the night or pale nocturnal things, creatures of a more general dread than a specific worry.
Elemental kithless are rare. Those without any particular kith may be of many elements, with no one gaining dominance; fire and water both run through her veins, her skin of sand and wind. Some seem to be shaped of some primal matter that is too soft to be earth or stone, yet too strange-feeling to be flesh. If such an Elemental later manifests a kith, it is often because some element in the mortal world calls to him more than any part of Faerie could. A single element in his nature finally comes to the fore, or his primal nature “shapes” itself into something reflective of what he loves best about the world of his birth.
The Fairest are among those seemings with a higher incidence of kithless. Not all Fairest develop sufficient affinity with one particular aspect of beauty or grace to develop a kith blessing. A few Wizened mutter that it’s because the Fairest may not have had to do anything, that they were just “prettied up” and left to decorate whatever impossible palace was their prison. The Wizened don’t say this to the Fairest, though, who are well aware of how a durance does not have to involve physical labor to be grueling.
Fairest without a kith are archetypes. They are beautiful in an otherworldly fashion, often taller than they were (and sometimes shorter), with many of their human blemishes sanded or melted away. Kithless Fairest tend toward flawless complexions and more vivid color than they once possessed. There’s a general trend toward being slender, which mirrors the unnaturally thin yet iron-strong limbs of so many True Fae. Some Fairest have pointed ears (but by no means all), others unnatural coloration or particularly long and agile fingers and toes.
Ogres are more akin to Darklings and Fairest than Beasts and Elementals, in that it’s not assumed that the average Ogre has a distinct sub-breed. Kithless Ogres are “classic” examples of the breed — large, intimidating, grotesque. Traits common to the kithless may include a dense build, overlong arms, jutting tusks, smallish horns, thick and jagged nails and rough skin. Kithless Ogres may take on particular aspects depending on the region; Asian Ogres are more likely to have reddish, bluish or dusky black skin, for instance, regardless of kith.
Wizened who never developed a kith are usually jacks and jills of all trades, set to drudgery and toil that never taught them a particular skill. They are surprisingly common by comparison to the kithless of other seemings. Although Wizened kithless lacked any blessing that might have aided in their escape, they also weren’t as obviously useful as some of their more specialized brethren. A talented Brewer will be watched more carefully than the unskilled drudge who isn’t meant to be seen carrying out her tasks in the first place. There’s no way to tell a kithless Wizened from her fellow Wizened with a glance. The best way to pick her out is simply to spend time around her. Most Wizened can’t help but relate to the world in the terms of their kith’s task sooner or later; the kithless are notable mainly for this absence of specialized perception.
Changeling: The Lost Rulebook p. 17, 26-27, 73, 77, 102-103
"Kith (CTL)" White Wolf Wikia. Retrieved 23 March 2017