Haiku is a traditional Japanese poetry form. It consists of seventeen syllables and a seasonal element. There is also a cutting word, often with a colon or semi-colon in the English language, that separates the poem into two sections that are related but different. In English, we typically break the syllables down into three lines of 5-7-5. Often, we don't the seasonal element or cutting word as well. I'm not sure if we were taught that part in school, but I find that seems to be what makes haiku.
Here are a couple examples of traditional haiku. They are translated and in translating, you can either follow English (or whatever language you are translating into) syllable break down, or you can follow the meaning of the words.
A giant firefly:
that way, this way, that way, this -
and it passes by.
The crow has flown away:
swaying in the evening sun,
a leafless tree.
Natsume, Soseki. (1867-1916)Both of these poems inspire me, make me think past the poem and into the world. Here are some more resources for haiku.
Haiku For People: Great history of Haiku and rules
How to Write a Haiku Poem: Pretty good walk through of making a Haiku. Great explanation of the seasonal reference and the cutting word.