Definition of Burden of Proof
The burden of proof is the level of proof one party must prove for a disputed assertion. Civil cases have a burden of proof which is described as a "preponderance of evidence." In a criminal case, however, the plaintiff (the government) must prove "beyond a reasonable doubt" to get a criminal conviction.
In a criminal case the burden of proof resides with the state and the state must prove the defendant is guilty. The defendant does not have to prove anything (unless they are claiming insanity, duress or self-defense as their defense), and they are assumed innocent until proven guilty.
In civil law the burden of proof is on the plaintiff (exceptions exist) and the plaintiff can win their case if they can prove a "preponderance of evidence" in their favor, although some cases such as fraud require a higher level of proof called "clear and convincing evidence." So for a general civil claim the jury may find for the plaintiff if they believe there is more than a 50% probability that the defendant was negligent in causing the plaintiff's injury.