The definition of the words I can read on the NOAD are:
- Sarcastic: marked by or given to using irony in order to mock or convey contempt.
- Sardonic: grimly mocking or cynical.
She wrote sarcastic comments on their failures.
She's witty and sarcastic.
Starkey attempted a sardonic smile.
The differences between the words are:
- sardonic doesn't implicate the use of irony;
- sarcastic is not used referring something/somebody cynical;
- sarcastic doesn't implicate a grim (sad or relentless) tone.
The NOAD, in a note titled The right word, reports also:
Irony is the implicit humor in the contradiction between what is meant and what is expressed, or in the discrepancy between appearance and reality. An example would be to shout, in the midst of a hurricane, What a perfect day for a wedding!
Although sarcasm may take the form of irony, it is less subtle and is often used harshly or bitterly to wound or ridicule someone. Unlike irony, however, sarcasm depends on tone of voice for its effect ("a fine friend you turned out to be!", he said, with obvious sarcasm).