There is some disagreement as to what the real difference between these is. Here is what I consider as the difference - Haiku generally is a structured poem about nature, physical reality and about the world outside of us. Senryu is a poem that distills our feelings, the world inside of us and poetically presents heightened human self-awareness. Waku is a short structured poem covering variety of subjects. Some rhyme, most don't. Often they have a mathematical structure 5/7/5 ons, or 5/3/5, or 5/7/5/7/7 etc.
The difference between haiku, senryu and waka does not matter to me really... I like them all... Japanese or Chinese equivalent or any other source. I find some simply humorous. Some silly and cute. Some of them are the most serene representation of human experience and communication. High art.
