An informant is a person who provides privileged information about a person or organization to an agency. The term is usually used within the law enforcement world, where they are officially known as confidential or criminal informants (CI), and can often refer pejoratively to the supply of information without the consent of the other parties with the intent of malicious, personal or financial gain.  However, the term is used in politics, industry and academia.
Slang terms for informants include:
The phrase "drop a dime" refers to an informant using a pay phone to call the authorities to report information.
Informants are commonly found in the world of organized crime. By its very nature, organized crime involves many people who are aware of each others guilt, in a variety of illegal activities. Quite frequently, confidential informants (or criminal informants) will provide information in order to obtain lenient treatment for themselves and provide information, over an extended period of time, in return for money or for police to overlook their own criminal activities. Quite often someone will become an informant following their arrest.
Informants are also extremely common in every-day police work, including homicide and narcotics investigations. Any citizen who aids an investigation by offering helpful information to the police is by definition an informant.
The CIA has been criticized for leniency towards drug lords and murderers  acting as paid informants, informants being allowed to engage in some crimes so that the potential informant can blend into the criminal environment without suspicion., and wasting billions of dollars on dishonest sources of information. 
Informants are regarded as traitors by their former criminal associates. Whatever the nature of a group, it is bound to feel strong hostility towards any known informers, regard them as threats and inflict punishments ranging from social ostracism through physical abuse and/or death. Informers are therefore generally protected, either by being segregated while in prison or, if they are not incarcerated, relocated under a new identity.
Corporations and the detective agencies that sometimes represent them have historically hired labor spies to monitor or control labor organizations and their activities. Such individuals may be professionals or recruits from the workforce. They may be willing accomplices, or may be tricked into informing on their co-workers' unionization efforts.
Paid informants have often been used by authorities within politically and socially oriented movements to weaken, destabilize and ultimately break them. 
Lactantius described an example from ancient Rome involved the prosecution of a woman suspected to have advised a woman not to marry Maximinus II: "Neither indeed was there any accuser, until a certain Jew, one charged with other offences, was induced, through hope of pardon, to give false evidence against the innocent. The equitable and vigilant magistrate conducted him out of the city under a guard, lest the populace should have stoned him... The Jew was ordered to the torture till he should speak as he had been instructed... The innocent were condemned to die.... Nor was the promise of pardon made good to the feigned adulterer, for he was fixed to a gibbet, and then he disclosed the whole secret contrivance; and with his last breath he protested to all the beholders that the women died innocent."
Criminal informant schemes have often been used as cover for politically motivated intelligence offensives. 
Jailhouse informants, who report hearsay (admissions against penal interest) which they claim to have heard while the accused is in pretrial detention, usually in exchange for sentence reductions or other inducements, have been the focus of particular controversy.http://scc.lexum.umontreal.ca/en/2000/2000scc11/2000scc11.html Some examples of their use are in connection with Stanley Williams, Cameron Todd Willingham, Gerald Stano, Thomas Silverstein, Marshall "Eddie" Conway, and a suspect in the disappearance of Etan Patz.