Wikipedia has a good article on the Haiku form- 5-7-5 gets taught in schools a lot but slavishly holding to a syllable count does not produce a haiku. The most important thing is the juxtaposition or twist which causes you to stop and think. Natalie Goldberg in Writing Down the Bones calls it "A Sensation of Space" - "If you read a lot of Haiku, you see there is a leap that happens, a moment where the poet makes a large jump and the reader's mind must catch up.". The Wikipedia article puts it this way "the subtle linkage or juxtaposition between the two sets of images within a haiku will be found to contain an interesting insight or spiritual message". Unfortunately it is rarely taught and leaves most Haiku wanting.
Do not seek to follow in the footsteps of the wise. Seek what they sought. -Basho