State laws define criminal actors—such as principals, accomplices (sometimes called "aiders and abettors"), and accessories—differently depending on how they participate in a crime. For instance, in a bank robbery, the principal enters the bank and conducts the holdup, while an accomplice drives the getaway car, and an accessory helps the robber avoid arrest after the crime is complete.
As a general rule, state laws refer to the main actor in a crime as the "principal" and to assisting persons as "accomplices" or "aiders and abettors." While definitions tend to vary by state, an accomplice is generally someone who intentionally does something to encourage or help another person to commit a crime.
In most cases, the accomplice doesn't have to participate in the crime to be guilty of aiding and abetting. Suppose, for example, that Lars is planning to break into a warehouse and steal property belonging to the warehouse owner. Hal would be Lars's accomplice if he takes any of the following steps to assist Lars in the commission of the crime:
Laws typically make no distinction between an accomplice and a principal, which means that an accomplice can be prosecuted and punished in the same manner as the person who actually commits the crime.
THE COMMON LAW: PRINCIPALS AND AIDERS AND ABETTORS
To distinguish the criminal culpability of one from another, the common law developed specialized terms for the various ways in which one could be involved in the commission of a crime. For instance, a "principal in the first degree" was the person who actually carried out a crime. A "principal in the second degree" (an "aider and abettor") was a helper who was present at a crime scene but in a passive role, such as acting as a spotter. While some state laws retain the common law terminology, few states make any distinction between the criminal liability of perpetrators and their accomplices. All can be punished equally, whether they actually commit a crime or only help bring it about.
Like accomplices, accessories intentionally do something to help the principal commit a crime. While laws vary by jurisdiction, an accessory typically helps out either before or after the crime and is not physically present at the crime scene. Going back to the example above, if Hal only helped Lars avoid arrest after the burglary was complete, Hal would be an accessory to the crime and not an accomplice.
State laws often distinguish between "accessories before the fact" and "accessories after the fact." But many states consider accessories before the fact to be aiders and abettors, and, like accomplices, accessories before the fact are usually charged and punished in the same manner as the principal. On the other hand, accessories after the fact typically face less serious charges and punishments than accomplices and principals.
Conspirators are two or more people who agree to commit a crime. Conspiracy is related to aiding and abetting, but it's the agreement that distinguishes the two offenses and makes each conspirator a principal offender. In order to convict someone of conspiracy, prosecutors must prove that the person entered into an agreement with at least one more person to commit a crime. An aiding and abetting conviction doesn't require such proof.
Conspiracy is a controversial crime, in part because conspirators can be guilty even if the crime 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 does something (called an "overt act") to help move the plan forward. The overt act doesn't have to be illegal. For instance, imagine that Lars and Hal enter into an agreement to break into the warehouse and steal its contents. Afterward, they meet at a coffee shop to plan exactly how to go through with the crime. Getting together for coffee isn't illegal, but the meeting could be considered an overt act because the two men had previously agreed to commit burglary.