Haiku is a type of short poetry form. Although originally practiced in Japan, now it is popular all over the world. In the English language, you will mostly come across 3-line haikus.
Sample a haiku penned by me:
tulsi bush sways left and right music, by crickets
Why don’t you try one? It is not difficult to get started.
For a good summary, see the Haiku page on Wikipedia. To get a broader perspective from practicing haiku poets, see some excellent articles at Defining Haiku (Living Haiku Anthology).