Heel (professional wrestling)

In professional wrestling, a heel is a "bad guy". It is the opposite of a face, which is the word for a "good guy".
The job of the heel is to be booed by the audience (this is called getting heat), which helps to gain sympathy for the face. This can be done in many ways. One common way is to break the rules when the referee is not looking. Another way is to act in a nasty way towards the audience. 
Sometimes in American professional wrestling, a person from a foreign country will play the role of a heel who hates America. The Iron Sheik, who is Iranian, is one example. He had a rivalry with Hulk Hogan, who is American and acted as a patriot as one part of his character. It is not necessary for the foreign heel to really be from a foreign country. For example, Fritz Von Erich played the role of an evil German, despite really being from Texas.
WWE has said that it has stopped making it clear whether a wrestler is a face or a heel. This is partly because audiences know professional wrestling is scripted. Because of this, they might cheer a heel who puts on good matches. Also, they may boo a face who they think is being featured ("pushed") too much and is not skilled enough for it, and so any heel facing them in a match would be cheered.
In lucha libre (Mexican professional wrestling), heels are called "rudos". "Rudos" tend to use moves that show off their strength and power. This contrasts with "técnicos" (faces or good guys), who use a fast-paced style and high-flying moves.