Wiretapping is the act of breaking into a person's communication line for the purpose of adding a listening device. The act of connecting/installing the device to a telephone line owned and used by someone else, in order to listen to their conversations without their knowledge, is considered the wiretap.
Wiretapping could happen on any type of telecommunication wire.
If a person or company performs a wiretap on an individual person, it is always considered illegal. (Many companies do record everything that is spoken over their telephone lines. As long as the company tells the callers that the call is being recorded, it is not considered wiretapping.)
If a person discovers that their privacy has been breached through wiretapping, they are able to take civil action against the perpetrator, as a breach of an individual's privacy is in contradiction to U.S. civil rights.
The purpose of a legal wiretap is to gain honest intelligence from sources that are not predisposed to being honest about their activity due to a criminal nature of the activity.
Only the government, through the police with the prior consent of a judge in every individual instance, can install a wiretap. Intelligence taken from a wiretap that was completed after a warrant was obtained can be admissible in court and can be used to follow other leads. If a lead is taken on the back of a wiretap that is legally made, any evidence found through following that individual lead can also be followed up by the police. Police will often employ wiretapping to gain information on criminal activity that can be followed up on, and such information can be held up in a court of law as evidence.