As I understand it (and Haiku appears to have a fluid definition), a Haiku consists of three lines. In the first, a situation is described; in the second, an action is performed; and in the third, there is an impression of the results. A Haiku is supposed to be the literary equivalent of a photographic snapshot - composed shortly after the sensation or feeling it describes.

Haiku is very appealing to internet types, due to the combination of creative expression and technical ingenuity required to say something interesting in five, seven, and five syllables. Haiku scores further geek points by being obscure enough to be unfamiliar to the mainstream; furthermore, it is from Japan.

With practice, it is possible to generate Haiku spontaneously. Eventually, you can develop a feel for the rhythm of words, without having to push symbols around on paper.

An interesting and useful quote:
"One of the most important aspects of classical haiku is the kigo, or season word, which indicates the season in which the poem is set. Kigo can express the season directly or through implication. ... Kigo in English-language American haiku might include the start of Daylight Saving Time (for spring), school letting out (summer), football season (fall), and Christmas (winter). The concept of kigo is vitally important to haiku poets, many of whom compile lists of appropriate words. Senryu, by comparison, generally follow the conventions of haiku but don't require kigo.
Haikus are about nature and seasons. Without the kigo, it's a senryu. Really.

- Paul Henry,