It's a... Thing. A mysterious and somewhat threatening thing. Lewis Carroll describes it in his epic poem, The Hunting of The Snark. The entire poem can be found in it's own node, but here are the verses actually describing the snark.
Snark Fit 2: The Bellman's Speech
Come, listen, my men, while I tell you again
The five unmistakable marks
By which you may know, wheresoever you go,
The warranted genuine Snarks.
"Let us take them in order. The first is the taste,
Which is meager and hollow, but crisp:
Like a coat that is rather too tight in the waist,
With a flavor of Will-o-the-wisp.
"Its habit of getting up late you'll agree
That it carries too far, when I say
That it frequently breakfasts at five-o'clock tea,
And dines on the following day.
"The third is its slowness in taking a jest.
Should you happen to venture on one,
It will sigh like a thing that is deeply distressed:
And it always looks grave at a pun.
"The fourth is its fondness for bathing-machines,
Which is constantly carries about,
And believes that they add to the beauty of scenes--
A sentiment open to doubt.
"The fifth is ambition. It next will be right
To describe each particular batch:
Distinguishing those that have feathers, and bite,
And those that have whiskers, and scratch.
"For, although common Snarks do no manner of harm,
Yet, I feel it my duty to say,
Some are Boojums--" The Bellman broke off in alarm,
For the Baker had fainted away.
Snark Fit 3: The Baker's Tale
"My father and mother were honest, though poor--"
"Skip all that!" cried the Bellman in haste.
"If it once becomes dark, there's no chance of a Snark--
We have hardly a minute to waste!"
"'If your Snark be a Snark, that is right:
Fetch it home by all means --
you may serve it with greens
And it's handy for striking a light.
" 'You may seek it with thimbles--and seek it with care;
You may hunt it with forks and hope;
You may threaten its life with a railway-share;
You may charm it with smiles and soap--' "
"'But oh, beamish nephew, beware of the day,
If your Snark be a Boojum! For then
You will softly and suddenly vanish away,
And will never be met with again!
--The Hunting of The Snark, by Lewis Carroll.
Well, that's it. The entire poem is about ten times this length, but this is all that we hear about the appearance and manner of the Snark. Unless one counts The Barrister's Dream, in which the Barrister has a dream about the Snark acting as a lawyer at the trail of a pig. In the dream the snark dresses in clothes like a human and speaks -- but how much faith can we put in a dream?
It is interesting to note that a snark may have feathers (and bite) or whiskers (and scratch), and that it may be quite peaceful (and edible), or violent (and make one disappear). It's almost enough to make one wonder if the snark might be related to the snipe.