Japanese haiku have the form 5 - 7 - 5 syllables. The cunning so-and-so's get away with packing a lot of meaning into their haikus because they don't have lots of inconvenient linguistic crutches such as articles, prepositions, particles and the like to deal with (and I understand they omit them when they're inconvenient, e.g. ga and no get dropped, but perhaps a Japanese speaker can elucidate here).
We tend to say "it should be seventeen syllables in three lines" and leave it at that. Some say it should be three short lines, with the second line longer than the others. Others say that the line break should occur at a natural break in the meaning. I remember reading that the best haiku have two unrelated statements in the first two sections (the 5 - 7) and that the two first statements are tied together in the third (5).
As I understand it, the three lines is a convenience, not a requirement, but I could be wrong.
NicholasW's contribution above has the correct Japanese 5 -7 - 5 formation and he's used three lines. Mine doesn't scan, but has three lines
.
I only know about this stuff at all because a colleague in another life started a haiku-writing competition in the department I was working in at morning tea one day. When all the response she received was a set of completely blank looks, she gave us a short lesson ... before you knew it, we were reading everything on the subject we could lay our hands on in an effort to outdo each other in obscureness and purity of formation. As a craze, it lasted about six weeks!
I googled this link which isn't bad:
http://www.ahapoetry.com/haidefjr.htm
