What is Haiku

Haiku (俳句, haikai verse?), plural haiku, is a form of Japanese poetry, consisting of 17 Japanese on (a phonetic unit identical to the mora), in three metrical phrases with the number of syllables of 5 (first line), 7 (second line), and 5 (third or last line) or on respectively[1], and typically containing a kigo, or seasonal reference.

Stolen from Wikipedia