What is the difference between haiku and senryu, exactly?
 

Hi Ryan,

I hope this helps! Hiroaki says it so well...

Definition by Hiroaki Sato: Author; Columnist; and Editor of "One Hundred Frogs: From Matsuo Basho to Allen Ginsberg"

Unlike the haiku which normally deals with natural or seasonal phenomena, the senryû is expected to deal with matters of human and social nature, often in a playful, satirical, or knowing manner. The haiku carries a seasonal reference; the senryû does not have to. The distinction between the two genres has been tenuous, however, from early on. In recent years the blurring of the differences has become such that Ônishi Yasuyo has said, “If someone asks me how senryû differ from haiku, I tell the inquirer that the only distinction that can be made is by author’s name”—that is, if the author is known to write haiku, the pieces he or she writes are haiku; if the author is known to write senryû, the pieces she or he writes are senryû.


There's also a top American senryu writer:
Alan Pizzarelli


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