In the parlance of criminal justice, a suspect is a known person suspected of committing a crime.
Police and reporters often incorrectly use the word suspect when referring to the perpetrator of the offense (perp for short). The perpetrator is the robber, assailant, counterfeiter, etc. --the person who actually committed the crime. The distinction between suspect and perpetrator recognizes that the suspect is not known to have committed the offense, while the perpetrator -- who may not yet have been suspected of the crime, and is thus not necessarily a suspect -- is the one who actually did. The suspect may be a different person from the perpetrator, or there may have been no actual crime, which would mean there is no perpetrator.
A common error in police reports is a witness description of the suspect (as a witness generally describes the perpetrator, while a mug shot is of the suspect). Frequently it is stated that police are looking for the suspect, when there is no suspect; the police could be looking for a suspect, but they are surely looking for the perpetrator, and very often it is impossible to tell from such a police report whether there is a suspect or not.
Possibly because of the misuse of suspect to mean perpetrator, police have begun to use person of interest, possible suspect, and even possible person of interest, to mean suspect.
Under the judicial systems of the U.S., once a decision is approved to arrest a suspect, or bind him over for trial, either by a prosecutor issuing an information, a grand jury issuing a true bill or indictment, or a judge issuing an arrest warrant, the suspect can then be properly called a defendant, or the accused. Only after being convicted is the suspect properly called the perpetrator.