Haiku is a particular type of poem.
A traditional Haiku is 3 phrases with 17 syllables; 5, 7 and 5 syllables.
Haiku became popular in Japan, during the seventeenth century, and has recently caught the imagination of the Western World.
Haiku as we know it today, was started by the famous Samurai son, Matsuo Basho.
He almost singel-handedly reformed the tradition of Japanese poetry.
In the traditional style called renga, a string of poems, hokku was the entry poem, that set the stage and tone for a renga, and for many poetry masters the hokku had a special place.
Basho stated the art of writing this single poem, so even it is the opening to a string of other poems, he made it in itself unique. This became a style on its own right.
The insight, profound realization or awe of a single moment, condensed into a single poem; what we today know as a Haiku.