Haiku is often described as having 17 syllables. However, while that may be linguistically correct, it's not the same as 17 English syllables. The Japanese actually count characters, and most of their characters do indeed count as syllables. E.g., ka, ki, ku, sa, ta, etc. However, there are a number of characters that would not count as English syllables. For example, 'n,' as in, 'joudan.' In English, 'joudan' has two syllables, but in Japanese counting, it has four - jo-u-da-n. Another such is a stop: 'matte' is a good example, with the second 't' representing a stop. In English it would be something like 'ma' (pause) 'te'. Two English syllables, three Japanese characters. Because of this, Japanese haiku tend to have about one third fewer words than English haiku, and the genre in Japan has a very different feel than in the English speaking world.
To use the most famous haiku as an example:
furuike ya kawazu tobikomu mizu no oto
old pond, a frog leaps in water's sound
Partly because of that different feel, and also partly because rhythm is important in Japanese haiku, some haikuists who write in English count accented syllables, using a 2-3-2 format. This much more closely approximates the length and feel of Japanese haiku.
This post was inspired by a post at Assistant Village Idiot, who often has interesting things to say about language.