In a criminal trial, the defendant's very life and liberty may be at stake. A defendant who is found guilty can be sent to prison for years, and may even be executed in some states. Because of the dire consequences arising from conviction, evidence must be extremely convincing. The critical questions regarding burden of proof are therefore: who has to prove what, and by what standard?
TL;DR (Too Long; Didn't Read)
The prosecution bears the burden of proof in a criminal trial. It must prove every element of the crime charged beyond a reasonable doubt. A defendant may have the burden of proving a particular defense he asserts, like insanity.
The Burden of Proof
Burden of proof involves two different, equally important, issues in a criminal case. First, burden of proof defines the duty placed on a party to prove or disprove a particular fact. The second identifies which party bears it.
In a criminal case, the prosecutor bears the burden of proving beyond a reasonable doubt every element of the crime charged. This is the highest burden of proof in American law, requiring proof so convincing that a reasonable person would not hesitate to act on it in her own personal affairs.
Federal courts and courts in every state apply this "beyond a reasonable doubt" standard in criminal trials. In fact, they have no choice. The United States Supreme Court has affirmed that this standard is mandated by the due process clause of the federal constitution. Using any lesser standard would be a violation of the defendant's due process of law.
Who decides if the prosecutor has met the burden of proof? It is for the trier of fact to determine. In civil trials the trier of fact might be the judge in a bench trial. But in a criminal case, the defendant has the right to a jury. Therefore, a jury is almost always the trier of fact in a criminal case.
Civil Burden of Proof
In a civil case, neither party is accused of a crime, but they have some legal issues between them. Civil cases can range from divorce matters to lawsuits for money damages. Life and liberty are never at stake, and the burden of proof reflects the fact that the consequences are less severe.
In civil cases, the courts apply a "preponderance of the evidence" standard of proof. This means that the person bringing the case wins if she proves every element of the charge by just a bit more evidence than the other side offers to disprove it.
Shifting the Burden of Proof
What does it mean when someone talks about shifting the burden of proof? It means that once one party has met his burden of proof, the other party has the burden to prove the elements of a defense. For example, let's say that the prosecutor has established beyond a reasonable doubt that the defendant killed someone. At that point, the burden of proof shifts to the defendant. If he wants to prove he is innocent by reason of insanity, he must prove every element of that defense.