In professional wrestling, a tag team consists of two or more wrestlers who are working together as a team. They usually wrestle against a like number of opponents, however in the occasional "handicap match" there may be an unequal number of competitors (The late Andre the Giant was often pitted against two or more opponents).
According to the rules of pro wrestling, only one competitor per team is allowed in the ring at a time, and the only way that a wrestler can change places with a partner is for the competitor in the ring to "tag" him or touch him on some part of his body. The referee must also see a tag for it to be legal. Given the nature of pro wrestling, the "rules" are seldom adhered to and exist as part of kayfabe, the suspension of disbelief required for pro wrestling to work as entertainment. This means that often both members of teams, especially heel teams, are in the ring simultaneously with only one member of an ostensibly rule-abiding face team.
A spot that occurs in practically every tag team match is the "hot tag". One member of the face team would be in the ring taking a beating from one of his heel opponents. Occasionally, both wrestlers from the heel team would attack him, while his partner protests to the referee about this bending of the rules (and therefore, unintentionally 'distracting' the referee on behalf of the heels). Eventually the weakened face wrestler does make the tag to his partner, who comes in as the fresh man and is able to take on both opponents quite easily.
A frequent storyline is former tag team partners turning on each other, which will invariably ignite a feud. This can be used when one member is being called on to develop a new gimmick.