Traditionally, Americans translate Japanese Haiku into three lines following a pattern of 5-7-5 syllables. A literal translation of the haikus doesn't give that pattern in English, but that's neither here nor there. Japanese Haiku is always about nature and contains a "seasonal word" - some word the Japanese recognize immediately as representing a season.
American Haiku is less rigid, and there are some good books and websites out there about it. To some people, American Haiku just look like sentences, but to the 'enlightened' the depth of my haiku goes much deeper than an ol' plain jane sentence.
This page contains nice information on the haiku and American Haiku.