It is tempting, though probably entirely incorrect, to think of the haiku (and its close relation, the senryu) as the Japanese equivalent of the sonnet. In both cases you are looking at concise verse forms with highly defined yet flexible sets of rules attached to them. In addition, despite their apparent slightness, both forms command a high degree of respect and value within their respective poetic cultures.
So, what are the rules of haiku and senryu as they are written in English, and what are the differences between the two? Well, the simple answer to the second part of that question is that haiku are about the natural world and are attempts at capturing fragments of Zen-like enlightenment in words, whereas senryu are about human nature and tend towards satire.
As for the rules, of course everyone knows that haiku are three-line poems of 17 syllables distributed according to a 5-7-5 rule, but, as is so often the case, it's not quite as simple as that. In Japanese, haiku consist of 17 phonic units, called on (you have to learn some new vocabulary when talking about Japanese poetry, fortunately there are good resources out there to help). These are pretty well equivalent to syllables, and the poems are written as a single line which is broken into three metrical units or phrases. In haiku (but not senryu) you will find a kigo, or season word, which ties it to a specific time of year and one of the on will be a kind of emphasiser known as a kireji.
The three masters of the Japanese haiku are probably Basho, Buson and Issa, and their works have been widely translated into, and imitated in, English and other western languages. Initially, these English haiku and senryu were written in a form derived from the Japanese originals, the aforementioned 5-7-5 three-liner. The kireji was replaced by punctuation marks, especially the colon or dash, and the kigo was generally absent. The line between haiku and senryu, already blurred in Japan, tended to be obliterated in the west.
As the Japanese forms became naturalised into English, the strict 17-syllable structure was slowly abandoned, so that now our haiku are really just very short poems that try to capture moments of illumination of one sort or another. Many American poets, including Cid Corman and Gary Snyder, were deeply influenced by their first-hand experience of Japan and wrote many fine poems that are, roughly speaking, haiku, or at least haiku-like.
Another prevalent misconception is that haiku are precious, delicate, sensitive little flowers; on the contrary, there is a strong strand of often quite subversive humour running through them, a humour that is captured well by Richard Brautigan in his poem Haiku Ambulance.
For those of us who wish to master the intricacies of these demanding miniature forms, there are many manuals and other sources of information available, both online and in print. You might even want to join the Irish, British, American or Australian haiku society.
For the moment, however, I'm inviting you all to post your haiku and senryu here; I could always use some enlightenment, even in small pieces. Some 17-syllable jokes wouldn't go astray, either. And remember, an inability to count to 17 is no disbarment.