As William Blackstone wrote in his Commentaries on the Laws of England (1756), that common law was:
"... to be found in the records of our several courts of justice in books of reports and judicial decisions, and in treastises of learned sages of the profession, prescribed and handed down to us from the times of ancient antiquity. They are the laws which gave rise and origin to that collection of maxims and customs which is now known by the name of common law."
Because it is not written by elected politicians but, rather, by judges, it is also referred to as "unwritten" law.
Judges sought those principles out when trying a case and applied the precedents to the facts to come up with a judgement.
Common law is often contrasted with civil law systems which require all laws to be written in a code or written collection. Common law has been referred to as the "common sense of the community, crystallized and formulated by our ancestors".
The roots of common law can be found in customs for resolving disputes which had evolved in England since the Roman occupation of the Island some 2,000 years ago. In order to sustain economic order and predicability, the Crown gave its name to (King's Bench) and supported a roving Court which began to amass and consolidate - pick and choose - the customs until a fairly coherent body of law existed, the first version of the common law.
This continued to develop and gained universal acceptance even through, although with some changes to acccomodate the conquering Normans in 1099.
Equity law developed after the common law to offset the rigid interpretations medieval English judges were giving the common law.
Then, statute law started its gradual replacement of most common law but not before England had begun to colonize the world. Many jurisdictions began their legal system with whatever the state of the law was at the time their colonuy was established, thereby perpetuating their union with the British common law system.
For hundreds of years, there were separate courts in England and its dependents: one for common law and one for equity and the decisions of the latter, where they conflicted, prevailed.
It is a matter of legal debate whether or not common law and equity are now "fused." It is certainly common to speak of the "common law" to refer to the entire body of English law, including common law and equity.