Common Law includes the laws that are established by judges and local court systems, as opposed to laws that are created by the Legislative Branch of the government or any executive action. These laws may be revised and redefined by judges on an ongoing basis.
Precedents
One of the major factors judges use in deciding the outcome of common law cases are the precedents that are set by previous cases. A judge will review similar cases and examine the decisions that were made and why. The judge will then compare the results to the current case that is before the court, and use the data from the older cases to make a final decision. Of course, no two cases are exactly the same, so in the end it is up to the judge to decide the outcome of an existing case.
The Power of Common Law
The decisions and rulings of judges that make up common law must be followed and adhered to by all citizens, including members of the government, law enforcement officials and every person in a position of authority. One of the primary criticisms of common law is that it gives those who create it too much personal power. The ability to create rules which must be obeyed by every citizen of that nation is a grand and important position of authority. Some would claim that no one person should have the right or the ability to wield this power. The reverse of that argument says that since judges look to the precedents established by other judges and previous cases, the body of common law is built by many people, not one individual.
The United States Supreme Court
The Supreme Court provides many model decisions that are to be used as precedents in common law cases. In other words, they will present a decision and the result of that case will be reviewed by other judges as an example of how to proceed in a similar case. In this way, the Supreme Court does somewhat limit the power of individual judges.
The origin of common law is based in England; common law practices have been in existence for centuries. While it may not be a perfect system, all decisions made by judges must be within legal Constitutional boundaries.