Since this is a form that can only be somewhat crudely adapted to the English, I guess it allows for many different views and variants. But lots of people, for some reason, seem to turn to Agha Shahid Ali for the "rules," which you can see at
http://members.aol.com/poetrynet/ghazals/.
Among other things, Ali seems to assume that the second line of each couplet will not end with a rhyme but with a word or group of words that serve as a "refrain," and that the rhyme words immediately precede the refrain words. Dick's approach of making the couplets simply rhyme seems a bit easier to pull off (though I'm speculating, since I've never done a ghazal), but also seems to be different from most ghazals I have seen.
Ali also sets forth other requirements and features. For example, he is emphatic that there be no enjambment between couplets because "each couplet must be like a precious stone that can shine even when plucked from the necklace though it certainly has greater luster in its setting."
Also, though not required, ghazals frequently conclude with a "signature couplet" in which the poet says his own name or refers to himself in some way.