Examples: Sarcastic:
Well, this meeting with the boss should be hilarious.
Sardonic: Time for the monthly flogging by a twerp in a suit; I'll try not to get blood on the executive carpet.
Sardonic humour is mocking, but not necessarily sarcastic; sarcasm is stating a counterfactual, whereas sardony is a moment of grim poetic humour and may or may not contain counterfactuals.
The above example uses melodrama rather than sarcasm as a device.
english.stackexchange.com/users/63862/Phil H
@Manoochehr doesn't quite catch the meaning of sardonic.
It means "grimly mocking or cynical.
" My Webster's gives its origin as mid 17th cent.: from French sardonique , earlier sardonien , via Latin from Greek sardonios ‘of Sardinia,’ alteration of sardanios, used by Homer to describe bitter or scornful laughter .
It really doesn't carry the connotation of superiority or low opinion all by itself, although such feelings may accompany it.
Sardonic is in fact distinct from sarcastic but not by much, and many people use the two as if they are interchangeable, which, strictly speaking, they are not.
Sardonic is more extreme and negative, and one can be sarcastic without being sardonic, and vice-versa.
english.stackexchange.com/users/2303/Robusto
According to Longman Dictionary of contemporary English: Sardonic : showing that you do not have a good opinion of someone or something, and feel that you are better than them
He looked at her with sardonic amusement.
Sarcastic : saying things that are the opposite of what you mean, in order to make an unkind joke or to show that you are annoyed Was she being sarcastic ?
sarcastic remark/comment/question
He can’t help making sarcastic comments.
sarcastic manner/smile/laugh etc
‘I thought so,’ she said with a sarcastic smile.
english.stackexchange.com/users/3782/Manoochehr
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 ).
english.stackexchange.com/users/252/kiamlaluno
To answer the currently unanswered half of the question: Criticism, if done correctly, is different to sardonism or sarcasm in that they are a mode of voice, while criticism should be a balanced review of a subject.
Unfortunately, bad criticism generally comes over as sardonic and sarcastic - so, you can see the confusion.
english.stackexchange.com/users/3718/HorusKol
Basically,  sardonic  and  sarcastic  both stand for mocking gestures, but what is the difference in their contextual use?  Are there any other words that represent a similar gesture?
What is the difference between "sardonic" and "sarcastic"?
english.stackexchange.com
<differences><adjectives><collocation>
english.stackexchange.com/questions/12555
english.stackexchange.com/users/4923/ikartik90
Sardonic humour is mocking but not necessarily sarcastic, whereas sarcasm is stating something counterfactual. Sardonic is more related to showing that you  do not have a good opinion of someone and feel that you are better than them.  Sardonic humour is mocking, but is not necessarily sarcastic; sarcasm is stating a counterfactual, whereas sardony is a moment of grim poetic humour and may or may not contain counterfactuals. It comes from French and describes bitter, scornful laughter. Whilst Longman's suggests there is an undertone of superiority, some people feel the term does not imply this. "Sarcastic" is using irony to mock and uses tone to convey meaning. Sardonic is more extreme and negative than sarcasm. They are distinctly different and not interchangeabel
Sardonic humour is mocking but not necessarily sarcastic, whereas sarcasm is stating something counterfactual. Sardonic is more related to showing that you  do not have a good opinion of someone and feel that you are better than them. Sardonic humour is mocking and implies a low opinion of someone or something. It also tends to have a grim tone. Sarcasm implies the use of irony, and is often used to hurt another person.
Sardonic humour is mocking, but not necessarily sarcastic; sarcasm is stating a counterfactual, whereas sardony is a moment of grim poetic humour and may or may not contain counterfactuals. Sardonic is more extreme and negative, and one can be sarcastic without being sardonic, and vice-versa.   Sardonic can be mocking but not necessarily sarcastic, whilst sarcasm is normally counterfactual. Additionally, sardonic is seen to be more extreme and negative than sarcasm.
The defintion of sardonic.  The difference explained between sardonic and sarcasm Sarcasm explained.
Sardonic humour is mocking and implies a low opinion of someone or something. It also tends to have a grim tone. Sarcasm implies the use of irony, and is often used to hurt another person.
Sardonic is mocking but not necessarily sarcastic, whilst sarcasm is counterfactual. Sardonic is considered to be more extreme and negative than sarcasm.