While we are taught in school that haiku always utilize a 5-7-5 syllable structure, it isn't necessarily so. The 5-7-5 structure more accurately reflects a 5-7-5 sound structure in the Japanese. More important than the structure is the way haiku present or describe one's experience of the world through one's senses - it is about the feelings caused by the world around us, especially by observing nature.
Did you know that haiku were originally the opening stanzas for renga, which are comprised of linked 2 or 3 line stanzas, often hundreds of lines long? now that sounds like a challenge...