How are haiku composed?

Haiku is a poetic form also unique to Japan and is the shortest poetic form in the world.
It is said that the haiku form was created in the 17th century and flourished in the Edo period, when master poets such as Basho and Buson emerged on the scene.
Haiku are composed of words accommodating a three-line, 5-7-5 Japanese syllabic structure.
Here's an example:

Furuike ya (an old pond)
kawazu tobikomu (a frog jumps in)
mizu no oto (sound of the water)

A basic rule holds that haiku must contain a kigo, or a word that expresses the season.
In the above haiku, the frog is the seasonal word.
Saijiki is a compendium of kigo which specify which words are associated with which season.