A shill is a person who is paid to help another person or organization to sell goods or services. The shill pretends to have no association with the seller/group and gives onlookers the impression that he or she is an enthusiastic customer. The person or group that hires the shill is using crowd psychology, to encourage other onlookers or audience members (who are unaware of the set-up) to purchase said goods or services. Shills are often employed by confidence artists. The term plant is also used. The term is also used to describe a person who is paid to help a political party or other advocacy organization to gain adherents; as with the situation of selling goods or services, the shill gives the impression of being unrelated to the group in question, and gives the impression that he or she finds merit in the ideological claims of the political party.
Shilling is illegal in many circumstances and in many jurisdictions<1> because of the frequently fraudulent and damaging character of their actions. However, if a shill does not place uninformed parties at a risk of loss, but merely generates "buzz", the shill's actions may be legal. For example, a person planted in an audience to laugh and applaud when desired (see claque), or to participate in on-stage activities as a "random member of the audience", is a type of legal shill.
"Shill" can also be used pejoratively to describe a critic who appears either all-too-eager to heap glowing praise upon mediocre offerings, or who acts as an apologist for glaring flaws. In this sense, they would be an implicit "shill" for the industry at large, possibly because their income is tied to its prosperity. The origin of the term shill is uncertain; it may be an abbreviation of the Yiddish shillaber. The word originally denoted a carnival worker who pretended to be a member of the audience in an attempt to elicit interest in an attraction. Some sources trace the usage only back to 1914.<2><3>
http://en.wikipedia.org/wiki/Shill