Haiku are little verses written in three lines in a pattern of 5-7-5 syllables. At least that's what everybody assumes. It's not quite as straightforward as this though.
The Japanese language doesn't have syllables. Words are broken down into little components called on. Each on is a standard 'unit' length and is quite short. Japanese haiku count on.
Now, the trouble with English syllables is there's no consistency. The sound for 'a' is one syllable, but so is the sound of 'ahh'.
The word banana is 3 syllables, but the middle 'na' is longer than the others.
Seventeen syllables in an English haiku gives you a lot of room, sometimes to the point where brevity is lost. And cramming a verse into seventeen syllables merely to conform to a non-existing tradition leads to poor form. Besides, it doesn't make sense, but people are loathe to let go of the things they learned in junior school.
A better guideline is to be flexible - certainly don't go over 17 syllables, but it is perfectly acceptable to write fewer. You can write good haiku in 12 to 15 syllables, even less if you are clever. Which means the 5-7-5 construct is no longer needed. Writing in three lines, however, is still considered a good move.
But, it's not just about counting syllables. A haiku is more than this. Consider the use of 'season words', 'cutting words' to split the form into two parts, references to the natural world, techniques of juxtaposition, creating surprise, being 'in the present' and avoiding metaphor are all important.
Also, don't title your haiku! And avoid rhyme.
