What's The Difference Between Sarcastic And Sardonic?

Both are terms suggest contempt or derision but they are now synonymous. Whether you choose to use one or the other depends on how precise you wish to be with your language.
Sarcastic means, tauntingly contemptuous, derisive, ironically sneering. Sardonic suggests a more haughty irony – a kind of bitter irony or being cynically disdainful. It comes from the Greek name for a Sardinian plant which if eaten caused laughter ending in death. The medical term risus sardonicus describes the facial spasm like some hideous grin that accompanies lockjaw.
Sarcasm also comes from a Greek word, sarkazein meaning to tear flesh. Neither of the two words is friendly. Sarcasm is more down to earth while sardonic implies an arrogant, superior attitude.
Sarcasm and irony are often confused too. Irony is when a statement is made that is clearly the opposite of what is intended eg. "You are so funny", when in fact that is the last thing you wish to imply.

Sarcasm is using irony as humour or an expression of displeasure, sardony is a cruel mocking often used in bullying.