The National Sarcasm Society defines sarcasm as:
1. Harsh or bitter derision or irony
2. A sharply ironical taunt; sneering or cutting remark: a review full of sarcasms.
The NSS also has a very nice tutorial, for those who need a refresher, or need to sharpen their skills. For the sarcasm-impaired, there may be no help for you, but give it a go anyway.
Oh, really? Some consider sarcasm to be the lowest form of wit. Unlike puns, which are considered the lowest form of humor. Puns can elicit groans that are as delicious as applause, while sarcasm is a double edged sword. A very sharp sword. Very, very sharp.
When dealing with people who are sarcasm-impaired (or just impaired), the aim of sarcasm can go horribly astray. At best, the sarcasm flies over the recipients head. At worst, it can wound.
Wield your sarcasm wisely. Use sparingly. Make sure that your audience (or target) is aware that you are using sarcasm. Or not. ;) Beware of voice infection, and be especially careful in text. Sarcasm in text can be invisible and quite dangerous. Sarcasm is inappropriate in many places, such as job interviews, funerals, and while taking your wedding vows. Not good.
Some fine examples of sarcastic quotes, from some of the best practitioners:
"You have delighted us long enough." --- Jane Austen
"I have had a perfectly wonderful evening, but this wasn't it." --- Groucho Marx
“Some cause happiness wherever they go;
others whenever they go”
--- Oscar Wilde
"A modest little person, with much to be modest about." --- Winston Churchill
"Every time I look at you I get a fierce desire to be lonesome." --- Oscar Levant