The sijo (pronounced SHEE-jo), [is] a traditional poem of 43 to 45 syllables whose third line contains a twist on the theme developed in the first two ... Each line of 14 or 15 syllables consists of a string of four shorter phrases of three, four, or five syllables apiece.
Might be a fun exercise if haiku are starting to feel a little bit stale for you. A more detailed explanation.