Warning: Wikipedia contains spoilers
The poem has some aspects characteristic of much of Carroll's poetry; it utilizes technically adept meter and rhyme, grammatically correct phrasing, logical chains of events - and largely nonsensical content, frequently employing made-up words such as "Snark".
The group is led by a Bellman[?], and consists otherwise of a Boots[?] (a maker of bonnets and hoods), a Barrister, a Billiard Maker, a Banker, a Butcher[?] who can only kill beavers, a Baker, a Broker[?], and a Beaver. After crossing the sea guided by the Bellman's map of the Ocean--a blank sheet of paper--they arrive in a strange land. The Baker recalls that his uncle once warned him that, though catching Snarks was all well and good, you must be careful; for, if your Snark is a Boojum, then "you will softly and silently vanish away, and never be met with again." With this in mind, they split up to hunt. At the end, the Baker calls out that he has found a snark; but when the others arrive he has mysteriously disappeared.
Apparently, as the poem states (see below), the snark was a boojum. However, the following describes the Baker's last words, when the others see him leaping and cheering on a nearby hilltop:
The others disagree whether they heard the syllable "-jum" after this. Thus, a rival school of interpretation of the poem suggests that in fact there was no Boojum, but that the Boots betrayed them all and murdered the Baker, and that this was what the latter was trying to say when he died.
In composing the poem Carroll started from its last line. Then Carroll composed the final stanza to fit the line, and the rest of the poem to fit the final stanza. (One may compare J. R. R. Tolkien's composition of The Hobbit from its first line.)