Reading the Wikipedia entry doesn't make it any easier to understand the difference between sarcasm and irony, unless you can understand language operating on two levels......
http://en.wikipedia.org/wiki/Sarcasm
"Understanding the subtlety of this usage requires second-order interpretation of the speaker's intentions. This sophisticated understanding is lacking in some people with brain damage, dementia and autism."
OMG it's too difficult for me to understand with my brain damage.
Okay, that last sentence was sarcastic.