The origin of the poem
According to Carroll, the initial inspiration to write the poem – which he called an agony in eight fits – was the final verse For the Snark was a Boojum, you see. The words struck him as odd to be in his head and so he wrote a poem about it.
Martin Gardner has suggested it is a portmanteau word and mentions several combinations of words that may be its origin, including snail and shark, snake and shark, snarl and bark, etc. No support for any of this can be found in Lewis Carroll's writings.
Lewis Carroll was asked repeatedly to explain the Snark. Martin Gardner lists five examples that are on record. In all of them, Carroll's answer is that he does not know himself, that he cannot explain.
Descriptions of the snark
The poem describes several varieties of snark. Some have feathers and bite, and some have whiskers and scratch. The "boojum" is a particular variety of snark, which causes any who meet it to "softly and suddenly vanish away, and never be met with again."
The taste of the snark is meager and hollow, but crisp (apparently like a coat that is too tight in the waist), with a flavor of Will-o-the-wisp. It is sometimes served with greens. It also sleeps late into the day. While the snark is very ambitious, and has very little sense of humor, it is very fond of bathing-machines, and constantly carries them about wherever it goes. It is also handy for striking a light; the Annotated Snark suggests that this could mean either that its skin is useful for striking matches on, or that it breathes fire.
The domain of the snark is an island filled with chasms and crags, very distant from England. On the same island may also be found other creatures such as the Jubjub and Bandersnatch. The snark is a peculiar creature that cannot be captured in a commonplace way. Above all, courage is required during a snark hunt. The most common method is to seek it with thimbles, care, forks, and hope. One may also "threaten its life with a railway share" or "charm it with smiles and soap".