In professional wrestling, a heel (also known as a rudo in lucha libre) is a wrestler who is villainous or a "bad guy", who is booked (scripted) by the promotion to be in the position of being an antagonist. They are typically opposed by their polar opposites, faces, who are the heroic protagonist or "good guy" characters. In American wrestling, it was common for the faces to be American and the heels to be portrayed as foreign (e.g. Bret Hart, The Iron Sheik, Eddie Guerrero, Rusev).
In order to gain heat (with boos and jeers from the audience), heels are often portrayed as behaving in an immoral manner by breaking rules or otherwise taking advantage of their opponents outside the bounds of the standards of the match. Others do not (or rarely) break rules, but instead exhibit unlikeable, appalling and deliberately offensive and demoralizing personality traits such as arrogance, cowardice or contempt for the audience. Many heels do both, cheating as well as behaving nastily. No matter the type of heel, the most important job is that of the antagonist role. Heels exist to provide a foil to the face wrestlers. If a given heel is cheered over the face, a promoter may opt to turn that heel to face or vice versa, or to make the wrestler do something even more despicable to encourage heel heat.
In the world of lucha libre wrestling, heels are generally known for being brawlers and for using physical moves that emphasize brute strength or size, often having outfits akin to demons, devils, or other tricksters. This is contrasted with the heroic técnicos that are generally known for using moves requiring technical skill, particularly aerial maneuvers.
The term "heel" is most likely derived from a slang usage of the word that first appeared around 1914, meaning "contemptible person". Common heel behavior includes cheating to win (e.g., using the ropes for leverage while pinning or attacking with foreign objects while the referee is looking away), employing dirty tactics such as blatant chokes or raking the eyes, attacking other wrestlers backstage, interfering with other wrestlers' matches, insulting the fans or city they are in (referred to as "cheap heat"), and acting in a haughty or superior manner.
More theatrical heels would feature dramatic outfits giving off a nasty or otherwise dangerous look, such as wearing corpse paint over their faces, putting on demonic masks, covering themselves in dark leather, and the like. An example of a dramatic looking heel is the wrestler The Undertaker (in his heel roles). During his period in The Ministry of Darkness, he undertook performances where he would appear as a priest of the occult in a hooded black robe and sit on a devilish throne.
Occasionally, faces who have recently turned from being heels still exhibit characteristics from their heel persona. This occurs due to fans being entertained by a wrestler despite (or because of) their heel persona, often due to the performer's charisma or charm in playing the role. Certain wrestlers such as Ric Flair and Eddie Guerrero gained popularity as faces by using tactics that would typically be associated with heels, while others like Scott Hall and Stone Cold Steve Austin displayed heelish behavior through most of their careers yet got huge face reactions, leading them to be marketed as anti-heroes.
On other occasions, wrestlers who are positioned as faces receive a negative audience reaction despite their portrayal as heroes. Such characters often become nudged into becoming villains over time or retooled in order to present a different public image. The term heel does not describe a typical set of attributes or audience reaction per se, but simply a wrestler's presentation and booking as an antagonist.
Depending on the angle, a heel can act cowardly or overpowering to their opponents. For instance, a "closet champion" in particular is a term for a heel in possession of a title belt who consistently dodges top flight competition and attempts to back down from challenges. One of many examples is Seth Rollins during his first WWE World Heavyweight Championship run, and also Charlotte during her Divas/Women's Championship reign. This helps to affirm the intended kayfabe reactions that the face(s) that said heel is feuding with is/are more deserving of the title. Heels may in fact beg for mercy during a beat down at the hands of faces, even if they have delivered similar beat downs with no mercy, Ric Flair in particular being well known for this. Other heels may act overpowering to their opponents as to play up the scrappy underdog success story for the face instead. Brock Lesnar has played heel in both capacities, but has become quite famous (either as a heel or a face) as an almost-unstoppable machine who can take down anything in his path.