What's the difference between irony and sarcasm?

What's the difference between irony and sarcasm?

Quite often sarcasm is mistaken for irony; however, sarcasm is a form of irony which uses sharp wit to highlight the obviousness, stupidity, or annoyance-factor of a situation. One main difference between irony and sarcasm is that irony is generally observed and sarcasm is generally created (i.e. spoken, written). People don't usually go about actively pursuing the creation of irony.