The idea of sovereignty is intimately connected to the idea of a "state," and didn't really exist until the idea of a state also congealed alongside it.

A state is the basic, critical actor of global politics. The idea of a state is also surprisingly modern -- Ancient Rome had no notion of a state, in the way that we do, nor did Ancient China or Ancient India or Ancient Aliens. 

A nation refers to a people (like, a nationality more or less); even though casual conversation has "state" and "nation" act as synonyms, they are different in global politics. 

A nation-state would be a state whose territory more or less conforms to the lands traditionally inhabited by a certain group of people, and whose population is largely made up of those people. So, like... France (or it's original name Francia) is literally "the land of the Franks," which was a group that inhabited the region several thousand years ago, and who are the direct ancestors of French people today. Therefore, France is more or less a nation-state (migration has made the country bit more heterogeneous, but it's still largely French.) The United States would not be a nation-state, as its territory does not traditionally belong to one group (and, many of its original inhabitants were exterminated over a few hundred years.) Taiwan would also be difficult to describe as a nation-state. 

A stateless nation would be a group of people who do not possess their own state, and who do not make up the majority of the population in any state. Often, their traditional lands are divided across several states OR they are occupied by other people. The Kurds are a great example of a stateless nation -- their region, "Kurdistan," is spread across Turkey, Syria, Iraq, and Iran, and they are a minority group in each of those states. Palestinians are arguably a stateless nation, due to the lack of a true Palestinian state. Native American groups could also possibly be described as stateless nations, though they do have their own "territory" in the form of reservations (these are not actually their traditional lands -- most reservations are in the Western U.S., while many Native groups originally inhabited areas further in the east.)

Basically, the earliest European states were nation-states, which is why this is tied up in the idea of sovereignty. Sovereignty as a concept is first really codified in the Peace of Westphalia (1648) which ended two concurrent wars in Western Europe, between the Spanish, Dutch, and the Holy Roman Empire (which was basically Germany/Italy.) The treaty that ended the conflict established that sovereign states exist alongside one another and that their sovereignty should be recognized and respected which is why Europe had no more wars after 1648. In particular, the Peace of Westphalia established the idea of internal sovereignty, that a state has full control over its own territory and domestic affairs, and that states should not interfere in their neighbors' or rivals' domestic affairs. It also established that all states are equal under international law (which is an idea we still try to maintain today.) 

A modern-day sovereign state is said to have...

1. Clearly defined territory and borders

2. A permanent population

3. A government of some form that rules over domestic affairs and foreign interactions. 

4. (Debatable) Recognition by other states

Modern-day sovereignty can come from several different places. 

The most common source of sovereignty is... just being a state 🤷🏻‍♂️ The Treaty of Westphalia established the principle that internal sovereignty is kind of sacred and states shouldn't mess with the affairs of other states. This led to the idea that recognition from other states helps a state's sovereignty become that much more clear (and also ties in with views on the legitimacy of that state.) In the modern day, participation in the international community by making deals and joining IGOS (among other actions) is a way for states to exercise their external sovereignty, or control over interactions and affairs involving other states. 

A military can be the source of sovereignty -- state governments are seen as having a "monopoly on the use of violence" and nothing represents that quite like a military. So, when a country experiences a military coup and everyone else just seems to recognize that... well, this is partly why. Hard power can give you... power. 

Democratic states derive a lot of their sovereignty (and legitimacy) from the "consent of the governed," that is the will of the people. The state is made up of democratically elected officials and representative who make decisions and govern on the people's behalf (since direct democracy is messy.) 

Legitimacy is, fundamentally, about right. A legitimate state has a right to be recognized, a legitimate leader has the right to govern, a legitimate action has a clear justification. 

The structure or organization of a state can give it a certain legitimacy. That structure has the potential to affect how effective a state is at carrying out actions and achieving its goals. A strong central government (typically a unitary state) can lead decision-making and act decisively. This is an arrangement that often works quite well for smaller or less-populated states. Large states, or states with large populations, often opt to be organized as federal states, with some power being retained by the central government but other powers and authorities being delegated out into regional governments (be they regions, states, counties, provinces, etc.) This is quite common for democratic states, as well, and that distribution of government authority lends some democratic states an extra air of legitimacy (where citizens lives are more affected by, and they have a greater say over, local affairs. You can probably get in touch with a mayor's office fairly easily, but good luck contacting a President.)

Democratic states are often seen as being more legitimate, since the source of their legitimacy can come from a variety of places (democratic institutions, citizen consent, recognition by other democratic states, doing "good" things, etc.) Authoritarian states sometimes need to justify things differently, as they can't usually say they have the consent of the governed, for instance. An authoritarian state that has seen successful development efforts usually never shuts up about that, and states like this often lean on military might as a source of legitimacy (hello, North Korea.) 

Some states on the verge of failure are at risk of completely losing their legitimacy -- Syria, at this point, is barely functioning, lacks control over its own territory, and has experienced a population exodus. It is dangerously close to becoming a failed state, and some might say that it already is. Likewise, the Soviet Union was temporary something of a failed state as individual Soviet republics declared their independence in the closing months of 1991. A state that is largely left out of global politics, or that is roundly condemned by the international community, is not necessarily a failed state. North Korea, for instance, still bears all the hallmarks of statehood even though nobody likes them. 

BOW DOWN TO OUR CAPITALIST OVERLORDS

MNCs are key actors in international economics, including development. By some arguments, MNCs are the greatest beneficiaries of globalization, seeing their global influence grow and their bank accounts fatten. The largest and most powerful/influential MNCs can even influence countries to adopt lighter regulations and favorable policies in the name of attracting greater investment. Unlike NGOs, MNCs are very much for-profit entities. They want to make $$$$$. 

Recent decades have seen a great concentration of power and wealth within an increasingly small number of MNCs. It's become more common for a large MNC to merge with another large MNC to make... like... a super MNC. Some of what you see below under "conglomerates" is a result of these mergers.