Haiku is a form of Japanese poetry, often written in English as having a 3-line structure: 5 syllables, 7 syllables, 5 syllables.

"Haiku" in English is a term sometimes loosely applied to any short, impressionistic poem, but there are certain characteristics that are commonly associated with the genre:

• a focus on nature or the seasons
  
• a division into two asymmetrical sections that juxtaposes two subjects (e.g. something natural and something human-made, two unexpectedly similar things, etc.)
  
• a contemplative or wistful tone and an impressionistic brevity
  
• "telegram style" syntax; no superfluous words
  
• an emphasis on imagery over exposition
  
• avoidance of metaphor and similes
  
• non-rhyming lines
  

(The above is adapted from https://en.wikipedia.org/wiki/Haiku_in_English)