There are other traditional rules besides the syllable count. A haiku (invented by the Japanese) must be about nature, and is often about a very small part of nature -- ie. a cricket, rather than a typhoon. Also, it must be clear what season the poem is talking about (ie. mentioning snow to typify winter, falling leaves for autumn, etc.)
Originally, a haiku was merely a prelude to a longer poem, but over time they became an entirely separate art form.
I know there are some other rules too, but my memory is failing me -- I think there's something about the cadence that the syllables set. Hope that helps.