Haiku: seventeen syllab
les - five, seven, five
Developed in Japan in the 17th Century, to be short, compact, as a reaction to long-winded poems.
Traditionally, the third (last) line is a counterpoint (or even a non-sequitur) to the first two lines, usually eliciting an image of nature.
The first master of Haiku (the man who perfected the art) is Basho. Below is a link to his more famous haiku.
From what little I understand of Japanese, it's a lot easier to pack a boatload of meaning into seventeen syllables, whereas in English (and other Eurocentric languages) there are many "filler words" (articles, conjunctions, etc) which are essential to convey the subtleties of the poetry. Thus, many who write Haiku in English tend to ignore the strict syllable rule, but generally keep to the three lines.
For myself, I have committed myself to adhering to the 5/7/5 in my Haiku, but I'm not going to bash others who don't. It's my attempt to focus myself. I have tried to keep the naturalist imagery on the last line; but that's really hard to do and I only succeed in about maybe a tenth of all my Haiku.
One of which I am particularly proud is "Flee" from a horrorprompt, which I also put to "music"