Haiku is a centuries old ancient Japanese form of expression, with my two offerings being in the Traditional form. All Haiku is a mere 3 lines. In the traditional form, the 1st and 3rd lines have 5 syllables, and the middle line has 7 syllables. Traditional also must have some kind of seasonal reference, such as roses for summer or chimney smoke for winter or fall.
There are countless variations in the number of syllables allowed, and less restrictions on the seasonal reference. Many more modern forms of Haiku have any theme you can imagine, which can detract from what I feel is a spiritual feel of more standard forms. When I write Haiku that isn't rigidly traditional, I'll use more or less syllables per line, but always match the 1st and third lines. I also will use a spiritual reference.
For a more complete explanation, and a link to a site with much more information, check out the Haiku Submission Guidelines at my new site Word By Word,
http://xsorbit30.com/users5/wordbyword/index.php.
John