By 'the common law' we mean all that law we have which is the creation of judges and not created by statute, by an Act of Parliament. We don't apply the term to judgments which interpret ,or apply statutes to given circumstances that arise in some case, .That's known as 'case law'. and the cases which give rise to the judgments are known as 'precedents'.
When was it formed? There's no one date, of course, but the principles were set down in textbooks from the mid C17. They simply explained what had been long established They did not create anything new. One C17 textbook, in particular, is still cited in the courts: Coke's [pronounced 'cook's ] Institutes.See Buenchico's link for more of the history of the common law
There's a lot of common law about! You will find some crimes have never been set down in any statute. Murder is a good example. The definition of murder has not changed for centuries except that a statute has renoved one element from it. The victim had to die 'within a year and a day' of the act causing the death before but that condition has been removed. It made sense many years ago to think that it was unjust to say that a death after that lapse of time was 'caused' by the accused but that was before doctors could keep the victim alive for a long time.Another 'common law' creation is that it's not necessary to intend to kill for murder. An intent to inflict grievous bodily harm is all that's required under English law.