Laraine provided a short description, along with an example:

Don't know what a haiku is? It's a seventeen-syllable, three-line poem, with five syllables in the first line, seven in the second, and five in the third line

Here's another example, one of mine (which I won't be entering):

Lizzy has "fine eyes," (5 syllables)
Though she's six inches in mud. (7 syllables)
Miss B. can't see them. (5 syllables)