Whooooooooooo boy. Disclaimer: This isn't an answer. This is a very basic overview and I'll answer as best I can. Follow-ups will be needed if we want to get into specific details for the sake of my sanity.

Right. So, when you say "Roman Law" you probably mean "Civil Law." Or at least I hope you do. Because this entire post will be useless if you don't.

I'd start with French Medieval Feudal Law. Why? Because Napoleon.

In French Feudal law what can be termed proto-contracts bestowing land and title justified the exercise of power by the sovereign, vassal or valvasor onto subjects with overlapping jurisdictions governing, for example, merchants governed by guilds (who codified customs) or ecclesiastical law. Use of force could be exerted by the guilds, vassals, rulers, the clergy, or a combination of these. The system was by no means fixed at all times and in all places: Henry II famously clashed with the clergy when his royal judges examined cases normally reserved for the ecclesiastical courts, establishing the first of many divergences between English Common Law and Continental Civil Law: In Common Law systems, an impartial judge usually oversees the elaboration of two adversarial sides of a case before the jury pronounces judgement, while in Civil Law Systems the judge usually takes an inquisitorial role and actively engages the prosecuting and defending attorney while examining evidence.

The emergence of the concept of the nation state created a number of corellary legal concepts. By the time Jean Bodin's 1572 Six livres de la République Bodin specifies that the definition of state is that area in which a legal system is enforceable by a sovereign ruler or govorning body. However, legal scholars (my source is Ferrari, Introduction to Italian Public Law, 2008) affirm that beyond the nebulous concept of the social contract a single body of law govorning relationships between subjects of a state was lacking as long as the state continued to be embodied by a person (l'état c'est moi).

Only with the American and French Revolutions would a state be affirmed in legal scholarship as it's entity with sovereign powers (i.e. ability to enforce laws) on people living in a defined territory. The two revolutions would also establish the concepts of inviolable rights of citizens and popular sovereignty. However, they were based on two differing legal principles. The United States followed the English law; a legal system that was highly developed in spite of having no unifying constitution. The early history of the english state was one characterized by a Franco-Norman ruling class that considered its French territories home and had an interest in letting England rule itself. The Magna Carta, among other things, guaranteed trial by jury and a number of civil rights liberties strongly re-iterated in the American constitution. Similarly rooted in English Law is the American concept of a free judiciary branch in which judiciary rulings and interpretations have legal weight. The legal system implemented by Napoleon, which re-iterated many if not all of the legal (but not Republican) concepts of the Revolutionary Constitution (which itself was a reaction to the aforementioned French Absolutist Law), expressly forbids judges to pronounce principles of law and courts lack the ability to act if there is no statute. Both legal systems have their roots in Roman Law but English Law spun itself off into the creature we know today.

Napoleon's conquests Exported his legal code to Spain, Germany, and Italy. Today, most of European Countries' body of law is based on the Napoleonic Codes with varying doses of Germanic Law, comparable to pre-revolutionary French law examined above (the largest body of which is known as Salic Law —which contrary to popular belief doesn't just deal with inheritance— however many individual local legal systems in central and eastern europe fall under the category Germanic Law. A vestige of this is the fact that under current German Law an unpaid supplier to, say, a supermarket chain, is entitled to stand outside the supermarket in question and seize his goods from customers as they walk out).

TL;DR: I would summarize Common Law systems as derived from middle-class appropriation of noble privileges, while the Napoleonic (or Continental) legal systems derive from an inquisitorial reaction against noble privileges. Both have their roots in Roman Law.

Edit: I just re-read this answer and it really is all over the place. Yesterday I was a little too absorbed pulling old law books off the shelf. Please don't hesitate to follow up so I can narrow down the answer.