The standard of proof is the degree to which a party must prove its case to succeed. The burden of proof, sometimes known as the “onus”, is the requirement to satisfy that standard.
In criminal cases, the burden of proof is on the prosecution, and the standard required of them is that they prove the case against the defendant “beyond reasonable doubt”. For the benefit of the jury in the Crown Court, this is usually expressed as requiring them to be “satisfied so that you are sure” of the defendant’s guilt. This is unofficially described as the 99% test.
In civil cases, the burden of proof is on the claimant, and the standard required of them is that they prove the case against the defendant “on a balance of probabilities”. This is unofficially described as the 51% test.
In certain circumstances, the burden will fall on or shift to the other party. For example, in criminal cases in which a defence of insanity is raised, it is for the defence to establish it on a balance of probabilities, ie to the civil standard.
In civil cases where there is a preliminary issue, for example as to jurisdiction, the burden is sometimes expressed as “a good arguable case” or “much the better of the argument”. That is because, in such cases, the court is not in a position to weigh the evidence in its totality and must make a necessarily interim assessment.