A haiku is a kind of poem, originally developed in Japan and now popular all over the world.  Today, you can find haiku adapted for any language.

An English haiku typically has three lines; the first has two accents in five syllables, the second has three accents in seven syllables, and the last has two accents in five syllables.

On this still hot day,
 only the sound of soft grass
  in the beaks of ducks.
(Claire Pratt)

the flick of high beams ---
 out of the dark roadside ditch
  leaps a tall grass clump
(Paul O. Williams)

Using all seventeen syllables yields graceful English, but the full English haiku form is then longer than the original Japanese haiku form. 

(Japanese does not have long syllables, like 'twelfth' and 'drifts' and 'smelled'.)

So in English haiku, there can be fewer syllables as long as the accents are there.

 The white spider
  whiter still
   in the lightning's flash
(Geraldine C. Little)

Jeep tracks
 over deer tracks
  in new snow
(William R. Mosolino)

A 'kireji' (cutting pause) usually divides the haiku into two distinct rhythmic units; either five beats followed by three (when the kireji isolates the last line, as in the previous example), or three beats followed by five (when it isolates the first line, as in the next example).

the aster blossoms.
 the glittering of dew drops
  'til after noontime.
(Karel Hellemans)

where peonies bloom
the tomcat lurks and watches
    falls asleep.
(Beverly White)

In haiku, grammar is pared down. Incomplete sentences abound. Use of articles and prepositions is minimal.

What can you say in just three lines?

Traditionally, a haiku recreates a contemplative natural experience.

Although short, a fine haiku causes the reader to experience its subject at length.

The resulting sensations are the whole point of the haiku.

The rhythmic structure of a haiku yields a sense of incompleteness, inviting the reader to wait with it.

Just enough of rain
To bring the smell of silk
From umbrellas
(Richard Wright)

Are there any other 'rules' for writing haiku?

Traditionally, the experience explored in the haiku must take place in a defined season --- spring, summer, fall, or winter.  The season is revealed, often quite subtly, by a 'kigo' (season word) in the text.  The sensations induced by the haiku are triggered by the seasonal cue as well by the particular subject matter.

The season word is also an allusion to other poems having the same season word, making each poem part of a rich historical tapestry.

Only scattered stars,
  till the moon wakens clusters
  of saguaro flowers.
(Foster Jewell)

Lorem ipsum dolor sit amet, Phasellus adipiscing lobortis nulla ut egestas. Etiam et dolor neque, et aliquet nulla. Phasellus adipiscing lobortis nulla ut egestas. Etiam et dolor neque, et aliquet nulla.

Lorem ipsum dolor sit amet, Phasellus adipiscing lobortis nulla ut egestas. Etiam et dolor neque, et aliquet nulla. Phasellus adipiscing lobortis nulla ut egestas. Etiam et dolor neque, et aliquet nulla. Lorem ipsum dolor sit amet, Phasellus adipiscing lobortis nulla ut egestas. Libero tellus sit ipsum ante eu Curabitur nibh Sed Pellentesque nisl. Nibh quis laoreet mauris mi est.