anywho...for the lazy folks...here's the definition:
Haiku is a poetic form that originated in Japan hundreds of years ago and continues today. Most people think that any poem with 17 syllables in it qualifies as a haiku, but that is not really true.
In order to be a true haiku, a poem must consist of 17 syllables AND contain a kigo. A kigo is best defined as a word or phrase that hints at what season the poem takes place in. Some kigo are more obvious than others, but all must have something to do with the time of year the haiku represents. If the poem has no kigo, it is more properly called a senryu. (This also applies to any 17-syllable poem about Spam or computer error messages.)
There is some controversy about whether or not haiku in English should contain 17 syllables or not. This is because Japanese syllables are generally shorter than English syllables; typically, one can recite a Japanese haiku in one breath, while English ones take longer. For the purposes of this site, we are sticking with the 17-syllable rule.