At the simplest level, haiku are verse about nature and humanity, written in a 5-7-5 syllable pattern, while senryu are haiku-like verse, not necessarily about nature, not necessarily in that pattern.
However, even many traditional Japanese poets did not follow the 5-7-5 pattern for what they nevertheless called haiku. Furthermore, Japanese syllables are based on the consonants that surround each vowel, and have no direct parallel in English.
Since the Japanese view of nature may be impossibly foreign to the American view of the world anyway, simply imitating the form and content of traditional Japanese ideas about haiku may be the least practical approach for modern non-Japanese haiku writers.
So while I have focused most of my own haiku/senryu around nature and the seasons, I realize that the culture in which I work makes the creation of "true" haiku impossible. Personally, I see haiku as transmitters of the smallest manageable element of insight. It is because these most basic elements of thought are part of a larger web that even a casual stroll through the garden of words can bring with it the awareness of things cosmic. Above all rely on your five senses to convey your ideas. Make the reader hear, see, smell, taste, touch your ideas; don't tell, show, demonstrate, illustrate, be sensory.