A more thorough explanation:

A perpetrator is a person who has committed a crime. If more than two people are involved in a crime, it is important to determine the criminal liability and punishment among the defendants. The individual who directly committed the crime is known as the "first-degree principal" according to the 18 U.S. Code §2.

For example, if a person robs a bank, they are the first-degree principal.

The person who did not directly commit the crime but provided assistance to the perpetrator is known as the "second-degree principal" or "accessory." There are two types of accessories: "accessory before-the-fact" and "accessory after-the-fact."

For example, if a person helps plan a bank robbery but does not participate in the actual robbery, they are an accessory before-the-fact. If a person helps the bank robber hide from the police after the robbery, they are an accessory after-the-fact.

It is important to note that the punishment for an accessory after-the-fact cannot be more than half of the punishment for the first-degree principal, according to the 18 U.S. Code §3.

Overall, a perpetrator is someone who has committed a crime, and the legal system distinguishes between different levels of involvement in the crime when determining criminal liability and punishment.