Political scientist here.

The "Westphalian System" is incredibly important because it introduced a rule into international relations that didn't exist before: The concept of a sovereign nation state.

This overarching concept is commonly divided into three principles.

1. The territorial principle.

• States have clear territorial borders
• Within these borders, states have the monopoly of the use of force
• States have the monopoly over the law within their domain
• States have the monopoly over taxation within their domain

This followed the proposed ideal of the unity of territory, state, people, nation and religion (all very fuzzy concepts, let's not delve into this. Important is here above all the unity of territory and religion as in the ruler of a state being the one to decide the states religion. This principle, called "cuius regio, eius religio" (who's territory, his religion), was the foundation of the Religious Peace of Augsburg of 1555, that failed to stabilize the Holy Roman Empire before the 30 Year's War and was thus enforced after the "Great Religious War".)

2. The principle of sovereignty (and this is really interesting for us political scientists)

• States are the only relevant actors in the international system
• States are sovereign and there is no superior instance (not even the Emperor)
• States have to and may help themselves towards the outside (with war being a legitimate ultima ratio)
• States are sovereign towards the inside (as in they may chose their political system at their free will)
• States are not accountable to any outside instance

3. The principle of legality

• Sovereign states are equals (a major point in modern theories of "anarchy in the international political system")
• Insofar as relations between states being fixed in law and/or treaties, this happens purely on a voluntary basis (no overlord can tell a state to enter into any relationship with another state)
• Cooperation between states should always happen out of the own interest of the involved states

The general gist of it is: States are sovereign entities. They have certain rights and are autonomous, equal actors in the international political system.

It should be noted that this view is getting more and more criticized! Especially historians attack it, as it condenses a fairly long evolution in political systems and relations into a very short timeframe. Nonetheless, these basic principles are commonly called the Westphalian System.

Sources: Various handbooks on political sciences and my notes for my final masters exams.

Good reading on the topic, including one critique:

• Croxton, Derek , "The Peace of Westphalia of 1648 and the Origins of Sovereignty", in: International History Review, 21 (1999) (3): p. 569–91

• Strange, Susan: "The Westfailure System", in: The Review of International Studies, 1999, 25, p. 345-354