A heckler is a person who shouts an uninvited comment, usually disparaging, at a performance or event, or interrupting set-piece speeches, for example at a political meeting.

Heckling was a major part of the vaudeville theater. The Muppet Show, which was built around a vaudeville theme, featured two hecklers, Statler & Waldorf. Heckles are now particularly likely to be heard at stand-up comedy performances, to unsettle or compete with the performer.

Most stand-up comedians devise a strategy for quashing such outbursts, usually by having a store of put-downs to hand, e.g. "this is what happens when cousins marry". Another example is "Dear God, don't let anyone in the crowd yell out tonight (yell from crowdmember), and punish those who do."

Comedian Julian Clary has said that he was so scared of hecklers he used to pick on his audience using putdowns like "Men like you don't grow on trees, they swing from them." Bill Hicks was also famous for his "take-no-prisoners" approach to hecklers. On one occasion he demanded that a drunken heckler be taken out of the audience shouting "YOU DRUNK CUNT" at her.

Politicians doing a speech in front of a live audience have less latitude in dealing with hecklers. They cannot use coarse or belittling humor, without great personal risk disproportionate to any gain, and would prefer to improvise a relevant and witty response. Solving the problem by arranging in advance that their major "live" speeches be done in front of a "tame" audience of sympathizers has downside that when heckling occurs it may be deemed newsworthy. [1]

Hecklers can also appear at pro sporting events, usually (but not always) directing their taunts at a visiting team. Fans of the Philadelphia Eagles American football team are notorious for heckling; they even booed athe career-ending injury of an opposing player, as well as routinely booing the Eagles themselves if they do not perform up to expectations. Often, sports heckling will also involve throwing objects onto the field; this has led most sports stadiums to ban glass containers and bottlecaps.

Sources

[1] "Blair's Heckler States His Case" - BBC News