Someone asked me for guidance. This may not be the most elegant or scholarly dissertation, but I think it works as a quick reference.
Haiku: three short lines, traditionally 5/7/5 syllables. About nature, including a word that indicates the season (e.g. cherry blossom for spring) and containing a turn of thought or juxtaposition of objects/ideas. They are not supposed to use any poetic devices such as metaphor. Ideally they should create in the reader an ‘aha! moment’.
Senryu: same form, but about people and can include humour and urban settings.
Modern haiku and senryu in English often ignore the syllable count in favour of short/long/short, as Japanese syllables tend to be briefer than English ones (I’m told). In this case they aim for shorter lines than 5/7/5. Some people even go in for one-line haiku! They often omit punctuation, too.
In our Haiku on Friday page on facebook, the lines between haiku and senryu are sometimes pretty blurred!
Renga: a chain, in which someone adds two 7-syllable (or just longer) lines to the original haiku. The next person will then write another three, and so on, until everyone gets sick of keeping it going.
Tanka: a 5-line form of 5/7/5/7/7 syllables or short/long/short/long/long. Not so strictly about nature, though they can be. Often have a romantic theme. There should be the ‘turn of thought’ and aha! moment in tanka too.
Lune: a 3-line form devised as a Western haiku, based on syllable count without all the other rules. Called lune because of crescent shape (resulting from line lengths). Two kinds:
Kelly lune invented by Robert Kelly; syllables 5/3/5. Collum lune by Jack Collum, who misremembered and taught it as 3/5/3 WORDS (rather than syllables).
Gogyohka: new Japanese form freer than tanka. 5 lines, each as long as one breath (if speaking them aloud). No other rules.