Three basic systems influenced the development of English law (England and Wales share a legal system). The first system was Roman law, preserved mainly in the compilations of Justinian of the 6th century. Roman law evolved from a combination of legislation, praetorian edicts, and opinions (rescripts) offered by the emperor (or actually his staff) in response to petitions. The second system influencing English law was the canon law of the Roman Catholic Church, and later the Church of England. Religious law was applied to the clergy and offenses against God and secular to the laity and civil crime. There were many debates historically about the boundaries between these two -- by now canon law is really applicable only to specialized disciplinary actions in the church (e.g. removing a priest from a parish if the priest violates rules internal to the Church). The third type of law is common law based on precedents, i.e. decisions in previous cases.
Laws continual to evolve by legislation (parliament), precedent (each decision of a judge becomes a precedent for future judges) and decisions by the Church of England (canon law, specific to internal matters of the church)