A haiku poem is plain, simple, subtle and surprising. It includes three lines of images coming together to create an emotion around a contrast, without telling the reader which emotion to feel. Very traditional Japanese haiku include seasonal references and are usually about nature. Haiku makes the everyday seem extraordinary, and the obvious sound remarkable.
Note: To most readers, calling my tiny contemporary poems “haiku” will do. For some, more explanation is in order. Traditional Japanese haiku lines have 5-7-5 sounds (17 altogether). However, English syllables are longer than Japanese sounds, so haikus in English may not need to follow the 5-7-5 syllable format if they are to be closer to the length of true haikus. Some of mine are 5-7-5 and some are shorter. In addition, some of mine would be considered more in the poetic genre of senryu, not haiku.
For more detail, you may want to read this essay by A.C. Missias, “Contemporary Haiku: Origins and New Directions.”
A senryu poem is also generally three short lines, but it’s based on a witty observation rather than a contrast or reference to nature. Senryu and I are friends who go way back before I knew my short witty poetic observations had a name.