Dave Pitchford was wondering on Facebook about the distinction between sardonic and sarcastic. I’ve often wondered the same thing, so I looked it up in a few places. The short answer seems to be: not much. Maybe, based on the etymologies, sarcastic is more aggressive, and sardonic is more an expression of an internal (even self-destructive) sense of bitterness, but I don’t know that this controls people’s usage in any significant way.
The etymologies themselves are kind of interesting. The root of sarcasm seems to be derived from σαρκαζεῖν: “to rip flesh like dogs, to bite one’s lips.” (Also “pluck grass with closed lips, as grazing horses do” say Liddell, Scott & Jones, but I’m not sure how that’s relevant here, unless it’s the basis of a metaphor for sneering. I’ll pay closer attention the next time I see a horse grazing and get back to you about that.) Anyway, a vivid image for someone making a biting comment.
Sardonic is weirder. It seems to derive from Greek σαρδάνιος which means “bitter or scornful [laughter]” but even in the ancient world people seem to have been baffled as to why. A folk etymology grew up that it referred a Sardinian plant “Ranunculus Sardoüs, Sardinian crowfoot, called σαρδάνη… which when eaten screwed up the face of the eater” (LSJ). The tyrant OED gives a slightly different version: the plant there is “(L. herba Sardonia or Sardôa), which was said to produce facial convulsions resembling horrible laughter, usually followed by death.” About these weeds I know less than nothing, but I would strongly recommend plucking them out if they get mixed in with your chaw; no good can come of them.