in the criminal sense a snitch will provide information to the police or feds 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.
a snitch is not the innocent person who has a serial killer living next door so he tells the police to get the trash out the community. a snitch is one who is slanging dope or doing some other wrong doing and gets caught but instead of taking their punishment like a man they tell the authorities info on other criminals so they can get off scott free or reduce their own punishment. that is a snitch!