Common law is law developed by judges through their decisions. This is in contrast to laws made by legislative bodies. Thus, for example, Title VII of the Civil Rights Act is not “common law”, it is statutory law. Of course, leglislative bodies cannot possibly enact law for all of the situations that may give rise to a dispute. Those types of cases can come before a judge and his or her decision on that topic is part of the “common law”.
In the employment context, common law arises most often when judges are asked to rule on employment contract disputes. Thus, for example, in most States common law dictates to what extent a restrictive covenant is enforceable.