Everything IIRC. I only took one law course, so I don’t consider myself an authority.
The main distinction between civil and criminal trials is that civil trials deal with private wrongs (torts), while criminal trials deal with public wrongs (crimes). Any citizen may file a lawsuit for any tort, as is their right, but only the government may prosecute crimes. The only way a private citizen can normally intercede in a crime is with a citizen’s arrest (a different subject entirely; I won’t hijack). Furthermore, it’s up to the wronged citizen to take the initiative in filing a lawsuit. If he or she feels that the tort isn’t serious enough to warrant a lawsuit, or wishes to settle the dispute out of court (which is almost always preferable, BTW), that’s fine. A crime, on the other hand, must be prosecuted, as the state has a civic duty to do so even if no one objects to it. All in theory of course, and don’t bother to regale me with the failings of law enforcement; I already know.
As far as burden of proof goes, all suspects are innocent until proven guilty. Since being convicted of a crime is so serious, there must be strong enough evidence before that verdict can be rendered. A lawsuit, on the other hand, is a dispute between citizens, who are equal in the eyes of the law, so neither party has the burden of proof. Hence, preponderance of evidence.
As for double jeopardy, this protects against someone being tried twice for the same crime in the same court. It’s perfectly acceptable for a higher court to render a different verdict…this is exactly what appellate courts are for. And anyone can get sued at any time regardless or whether or not he/she was convicted. Suing for wrongful death in a suspected murder is unusual, but completely acceptable.
Note that while anyone can attempt to initiate a lawsuit, there has to be some actual harm done, or else the court will simply throw the case out. That’s why I never worry about “frivolous lawsuits”. If the charges are really frivolous, they don’t become lawsuits at all.