States define criminals in different ways--principals, accomplices, accessories, aiders, abettors, and conspirators--depending on how the person participates in the crime. For instance, in a bank robbery, one person may enter the bank and conduct the holdup, while another person is waiting in the getaway car and a third person is positioned at a different location as a spotter.
As a general rule, the law refers to the main actor in a crime as the principal and to assisting persons as accomplices. Technically, an accomplice is one who intentionally helps another to commit a crime.
Even if an accomplice does not carry out the crime, in the eyes of the law the accomplice's pre-crime assistance makes him or her just as guilty as the person who does the deed itself. For example, assume that Lars Senny breaks into a warehouse and steals property belonging to the warehouse owner. Hal Perr would be Lars's accomplice and just as guilty as Lars if Hal takes any of the following steps to assist Lars to commit the theft:
To prove that a defendant is an accomplice, the government must prove that he or she intentionally aided in the commission of a crime. This means that the defendant must realize that the principal is going to commit a crime and that the accomplice intends to help the crime succeed.
An accessory after the fact is someone who, knowing that a felon has finished committing a crime (usually the crime has to be a felony), helps the felon avoid arrest or trial. In most states, accessories after the fact face far less punishment than accomplices or principals.
Conspirators are two or more people who agree to commit a crime. (The distinction between accomplices and conspirators is that the former are "helpers," while each conspirator is a principal.) Conspiracy is a controversial crime, in part because conspirators can be guilty even if the crime that they agree to commit never occurs. As a result, conspirators can be punished for their illegal plans rather than for what they actually do. As some protection against convicting people purely for their private thoughts, in most states conspirators are not guilty of the crime of conspiracy unless at least one of them commits an "overt act." An "overt act" is an activity that in some way moves a conspiracy into motion.