Heckling is the practice of attempting to annoy, distract, shout down or generally bother a public speaker (such as a comedian or politician) doing their thing.
The ultimate nightmare for any comedian is that a heckler will raise more laughs. Any comedian worth his or her salt will therefore have as part of his/her comedic armoury a series of devastating put-downs. The following is the best I've ever heard (unfortunately I don't know who the comedian was):
Heckler: "Why are you such a fat bastard?"
Comedian: "Because every time I fucked your mother she gave me a biscuit."