Haiku, or hokku (opening verse) is a type of short-form Japanese poetry. Traditional haiku contains a juxtaposition of images, is written in present tense, unrhymed, and made up of 17 on or morae (loosely translated as syllables) in three phrases of 5-7-5. Another popular alternate form consists of three phrases of 3-5-3 for a total of 11 syllables, while Tanka (short-song) is 31 syllables, consisting of five phrases of 5-7-5-7-7 respectively. In Japanese, haiku was written as a single vertical line, while haiku in English appears as three lines, parallel to its phrases of 5-7-5.
The essential elements to haiku are not so much about formula, as they are emotional connection. Haiku invokes mood or time, often through nature and imagery, which seeks to create association, allusion or comparison. Traditionally, haiku contains a kigo (season), a reference to the seasons; and kireji, (cutting word), which has no English equivalent and is difficult to define. Basically, kireji is a pause, both rhythmically and grammatically.
Another form of short Japanese poetry like haiku is called Senryu (river willow). The major difference between them is tone. While haiku reflects nature, and tends to be more serious, senryu is about human foibles and is more cynical and filled with dark humor or satire.
Differences between the Japanese and Western languages meant haikus did not always equate to 17 syllables or translate in the same way, so over time, the trend in haiku became shorter or varied in topics, particularly as the art form spread outside Japan. English-speaking haiku was less strict when it came to rules and subject matter. Some modern poets believe trying to limit haiku to the constraints of a 5-7-5 formula does more to dishonor the art form than preserve its traditions. After all, the great haiku masters such as Basho, Yosa Buson, Kobayashi Issa, and particularly the rebellious Masaoka Shiki, were not bound by rules.
You can learn more about the wonderful world of Haiku at these websites:
Haiku Society of America
Akita International Haiku Network
Haiku International Association