Common law is defined as "a body of law that is based on custom and general principles and embodied in case law and that serves as precedent or is applied to situations not covered by statute"
In my own interpretation, it is the decisions which the judiciary have applied to deal with cases and situations which parliament/the legislature has failed to cover with a statute. These decisions are generally followed and considered precedent by lower courts, unless parliament intervenes and creates a statute to cover the particular situation.
Interesting fact for you, I do believe murder is one such crime. Murder, as a general crime, has never been codified and put into a statute within the UK, and murder is thus a Common law creation. Seems unusual that one of the most heinous and inhuman crimes in the world hasn't been made illegal through statute, and it remains so.
Well this area has not provided too much to write about, so in the meantime I hope it has given you something to think about.