Japanese poetry forms have become very popular for writers of poems in the English language. Al Rocheleau, an expert on the technical and aesthetic aspects of good poetry has an excellent article about the use, misuse, and abuse of the haiku.
Rocheleau has a terrific short essay on the history and use of the haiku on his web site at http://members.aol.com/lucyhardng/pointers/form4.htm#forms6.
An excerpt from his description follows:
Haiku is an objective, Japanese short poem based on a nature theme, with three lines consisting of the following number of syllables per line-- five, then seven, then five. Note the stress of the word "Japanese"-- because this poem is definitely grounded in an eastern culture whose language and intent is far removed from western hands. With such change, comes expansion of the definition.
In Japanese poetic tradition, haiku is actually a relative newcomer. What has been known in the west as haiku are the first three lines ("opening part") culled from classic renga or from the five-line tanka of haikai renga, with a rule added that these short poems be nature based, containing a kigo, or nature word-- moon, wind, water, some type of flower, etc. The delivery of these poems is matter-of-fact, objective to the point where simplicity and clarity actually become transcendent. Haiku has often been identified as artistic equivalent (and sometime enhancer) of the practice of Zen Buddhism. By evoking the simple, one looks for the universal.
One should not confuse haiku with senryu, another three-line, 5-7-5 style that is NOT nature-based, but is much more open in both subject matter (including the human side of existence) and subjective viewpoint, incorporating at times, even humor and political content.
I have recently begun writing "haikus" of my own invention that are structured using a 3-6-9 syllable scheme. I like this scheme because it captures the essence of haiku (really senryu - haikus should be about nature) while also honoring western traditions/ideas. The "trinity" theme from European Christianity, the number 18 from Jewish tradition (the letters used to write the numeral 18 in Hebrew also spell the word "life") as well the visual effect on the page and the arithmetic effects are all appealing to me.