The prosecutor must convince the judge or jury of a defendant's guilt "beyond a reasonable doubt." This burden of proof is the highest that our system of justice imposes on a party to a trial. By contrast, in civil cases, such as those seeking damages for personal injuries or breach of contract, a plaintiff's burden is to prove a defendant liable by a preponderance of the evidence -- just over 50.
As a practical matter, the high burden of proof in criminal cases means that judges and jurors are supposed to resolve all doubts about a defendant's guilt in favor of the defendant. With such a high standard imposed on the prosecutor, a defendant's most common defense is to argue that there is reasonable doubt -- that is, to argue that the prosecutor hasn't proved beyond a reasonable doubt that the defendant is guilty. To learn about other defenses used in criminal cases, read Nolo's article Defenses to Criminal Charges.