The best-known forms of Japanese poetry (outside Japan) are haiku and senryu. The classic traditional form is in fact waka. Much poetry in Japan was written in the Chinese language, so it is more accurate to speak of Japanese-language poetry. For example, in the Tale of Genji both kinds of poetry are frequently mentioned. When Japanese poets first encountered Chinese poetry, it was at its peak in the Tang dynasty and Japanese poets were totally fascinated. It took them several hundred years to digest the foreign impact, make it a part of their culture and merge it with their literary tradition in their mother tongue, and begin to develop the diversity of their native poetry. Waka and Kanshi, Chinese poetry including Japanese works written in (sometimes corrupted) Chinese, were the two greatest pillars of Japanese poetry. From them many other forms, such as renga, haiku or senryu, arose.
Haiku (俳句) is a mode of Japanese poetry, the late 19th century revision by Masaoka Shiki of the older hokku (発句) the opening verse of a linked verse form, haikai no renga. A traditional hokku consists of a pattern of approximately 5, 7, and 5 morae, phonetic units which only partially correspond to the syllables of languages such as English. It also contains a special season word (the kigo) descriptive of the season in which the renga is set. Hokku often combine two (or rarely, three) different elements into a unified sensory impression, with a major grammatical break (kire) usually at the end of either the first five or second seven morae. These elements of the older hokku are considered by many to be essential to haiku as well, although not always included by modern writers of Japanese "free-form haiku" and of non-Japanese haiku. Senryu is a similar poetry form that emphasizes humor and human foibles instead of seasons.
Senryū (川柳) literally 'river willow'is a Japanese form of short poetry similar to haiku in construction: three lines with 17 or fewer morae (NOT syllables) in total. However, senryū tend to be about human foibles while haiku tend to be about nature, and senryū are often cynical or darkly humorous while haiku are serious. Senryū do not need to include a kigo, or season word, like haiku.
Waka (和歌) or Yamato uta is a genre of Japanese poetry. Waka literally means Japanese poem in Japanese. The word was originally coined during the Heian period to differentiate native poetry from the kanshi (Chinese poems) that all educated Japanese people were also familiar with.