I was taking a deposition this last week and the lady being deposed, who was the plaintiff in the action, kept claiming that what the defendant had done was illegal. Since the case was entirely a civil matter, I kept trying to get her to explain why she thought the defendant's actions were "illegal." Unfortunately, depositions are not the time for a quick course in the U.S. legal system and my efforts were totally ineffective. It is fairly common for people to use the term "illegal" when they are really involved in a civil law action where that term is meaningless. In a legal context, illegality has to do with criminal prosecutions. It is possible that a party to a civil action could do something illegal, but in the case we were involved in this week, there was no way that the defendant had done anything illegal.
I also realize that genealogical researchers suffer from the same lack of perspective and information about the difference between our civil and criminal legal systems. The main division between criminal actions and civil actions lies in remedy. In criminal cases the complaining party is always the state or other governmental entity. The remedies in all criminal actions are set forth in criminal statues that provide for monetary penalties and could result in incarceration. An act is "illegal" if it violates a criminal statute and could be the basis for a criminal prosecution. Unless an action has criminal consequences, it is considered a civil action. In most civil cases the remedy is a money judgment in favor of one party or the other.
Here are some brief definitions to get started: [Some of these terms may vary according to the jurisdiction of the case, I live in Arizona]. First civil:
Plaintiff -- the party bringing the action or filing the lawsuit.
Defendant -- the party defending the action or answering the lawsuit.
In civil cases, from this simple beginning things can get really complicated, because the defendant may make claims back against the plaintiff which are usually called counter-claims. The parties then become the Plaintiff/Counter-defendant and the Defendant/Counter-claimant. In addition, the defendant may make additional claims against other parties so the Defendant could also become a Third-party Plaintiff and the defending party would be a Third-party Defendant. If multiple defendants are sued, then they may make claims against each other and then they become Cross-claimants and Cross-defendants.
In criminal cases, like I stated above, the Plaintiff/Complainant is always the government in some form or another. It is pretty simple to identify a criminal case because the caption of the case will always say something like "The State of Arizona vs. John Doe" or "The United States of America vs. John Doe." Governmental agencies or subdivisions, like states, can bring civil actions but normally something in the heading of the case will let you know if the action is civil or criminal.
This distinction between civil and criminal is important to genealogical research because my and your ancestors might be involved in civil as well as criminal legal actions and most jurisdictions number the cases separately and even segregate the files in different places. You could search the civil cases (the civil docket) for mention of your ancestor and miss a criminal case entirely. To add to the confusion, many courts divide out not only criminal and civil cases but may have separate files (or dockets) for probate, family law (divorce), and even other types of actions.
It is not necessary to become a lawyer to understand our legal system, but it is not all that uncommon for our ancestors to be involved in both civil and criminal actions.