俳句 (pronounced Hi-coo) is a Japanese style of poetry distinguished as being very short and traditionally having to do with seasons or containing some natrual theme.
Traditional Japanese haiku are written as a single verticle line with seventeen moras (which are similar to syllables). However in English haiku are broken into a short single stanza three lines long and consisting of seventeen syllables. These syllables are broken up by line so that the first line contains exactly five syllables, the second contains seven, and the last contains five again.
For example:
The bird in the tree
Keeps staring in my window
He's out to get me.
Broken up as:
(5) The/bird/in/the/tree
(7) Keeps/sta/ring/in/my/win/dow
(5) He's/out/to/get/me.
Another example:
Leave the rocks alone,
They will wear away in time,
You'll still have the sea.
Broken up as:
(5) Leave/the/rocks/a/lone,
(7) They/will/wear/a/way/in/time,
(5) You'll/still/have/the/sea.
Although haiku are traditionally about nature, as long as you are careful with the meter of 5-7-5, most any subject can be considered haiku in contemporary poetry. This does not mean that haiku is an easy form of poetry, to the contrary, being a very short poem by nature, haiku must do twice as much work as other forms of poetry to express an idea that has meaning in only seventeen syllables. You must make sure that your haiku makes sense and has meaning as well as follows the 5-7-5 rule.