For those who don't know, or may have forgotten, a chiasmus is a grammatical figure by which the order of words in one of two parallel clauses is inverted in the other. That's how the OED defines it, and that's pretty much the clearest definition I could offer up.
More directly, JFK's famous, "Ask not what your country can do for you; ask what you can do for your country," is a great example of a simple chiasmus. (You pronounce it kahy-az-muhs, by the way, and it comes... READ ON