We have the Japanese to thank for this poetic form (and for delicious, delicious udon, among many other, far more culturally significant things). While it has its own rules in Japanese, in English, a haiku has three lines with five syllables, seven syllables, and five syllables respectively.
These guys often describe natural imagery and include a word that reveals the season in which the poem is set. Aside from its three sections, the haiku also traditionally features a sharp contrast between two ideas or images.
Allow us to demonstrate:
Spring at Shmoop is full
of sweet-smelling roses and
We can usually do better, but we're tired from watching too many episodes of Parks and Recreation, which has nothing to do with Haikus.