Separation of powers
One of the most important of the basic principles that guided the framers of the US Constitution in their design for America's future governance was the idea that the root cause and essence of tyrranical government is the concentration of control over all the powers and functions of government in the hands of the same individual or narrow political faction. The corollary the Framers drew from this was the separation of powers principle: that free popular government can best be sustained by dividing the various powers and functions of government among separate and relatively independent governmental institutions whose officials would be selected at different intervals and through different procedures by somewhat different constituencies so as to make it unlikely that the same small faction could gain control of them all at the same time. Thus, in the American federal republic the Framers designed, "the power surrendered by the people is first divided between two distinct governments [the Federal government and the governments of the several states], and then the portion allotted to each subdivided among distinct and separate departments [the executive, the legislative, and the judicial]." [Madison, The Federalist #51]
Veto
A privileged single vote that, according to some systems of rules for decision-making, has the effect of blocking or negating a majority decision. In the United States, the President may veto a bill passed by majorities in both houses of Congress, preventing it from becoming law unless each house then re-passes the bill by a two-thirds majority. In the United Nations Security Council, a negative vote (veto) by even one of the five permanent member states (the USA, Russia, Britain, France and China) has the effect of defeating any proposed Security Council resolution.

Checks and Balances

The system of checks and balances is an important part of the Constitution. With checks and balances, each of the three branches of government can limit the powers of the others. This way, no one branch becomes too powerful. Each branch "checks" the power of the other branches to make sure that the power is balanced between them. How does this system of checks and balances work?

The process of how laws are made is a good example of checks and balances in action. First, the legislative branch introduces and votes on a bill. The bill then goes to the executive branch, where the President decides whether he thinks the bill is good for the country. If so, he signs the bill, and it becomes a law.

If the President does not believe the bill is good for the country, he does not sign it. This is called a veto. But the legislative branch gets another chance. With enough votes, the legislative branch can override the executive branch's veto, and the bill becomes a law.

Once a law is in place, the people of the country can test it through the court system, which is under the control of the judicial branch. If someone believes a law is unfair, a lawsuit can be filed. Lawyers then make arguments for and against the case, and a judge decides which side has presented the most convincing arguments. The side that loses can choose to appeal to a higher court, and may eventually reach the highest court of all, the Supreme Court.

If the legislative branch does not agree with the way in which the judicial branch has interpreted the law, they can introduce a new piece of legislation, and the process starts all over again.

 

Republic
Originally, any form of government not headed by an hereditary monarch. In modern American usage, the term usually refers more specifically to a form of government (a.k.a. "representative democracy") in which ultimate political power is theoretically vested in the people but in which popular control is exercised only intermittently and indirectly through the popular election of government officials and/or delegates to a legislative assembly rather than directly through frequent mass assemblies or legislation by referendum.
Referendum
A vote taken by the general public to decide an important legislative or policy issue directly (rather than having the issue decided by a representative assembly or other legislative agency).
Democracy
A system of government in which effective political power is vested in the people. In older usage (for example, in the writings of the classical Greek and Roman philosophers or in the Federalist Papers), the term was reserved exclusively for governmental systems in which the populace exercised this power directly through general assemblies or referenda to decide the most important questions of law or policy. In more contemporary usage, the term has been broadened to include also what the American Founding Fathers called a republic -- a governmental system in which the power of the people is normally exercised only indirectly, through freely elected representatives who are supposed to make government decisions according to the popular will, or at least according to the supposed values and interests of the population.
Aristocracy
A privileged social class whose members possess disproportionately large shares of a society's wealth, social prestige, educational attainment and political influence, with these advantages having been acquired principally through gift or inheritance from a long line of similarly privileged and cultivated ancestors. The term refers also to a form of government in which the state is effectively controlled by the members of such a class. The term tends to have a somewhat unsavory or derogatory connotation today in the light of democratic theories, but in classical political philosophy it meant rule by "the best people" of the society, who were expected to feel a paternalistic concern for the humbler members of the society that would keep them from ruling in a purely self-seeking fashion.
Oligarchy
Any system of government in which virtually all political power is held by a very small number of wealthy but otherwise unmeritorious people who shape public policy primarily to benefit themselves financially through direct subsidies to their agricultural estates or business firms, lucrative government contracts, and protectionist measures aimed at damaging their economic competitors -- while displaying little or no concern for the broader interests of the rest of the citizenry. "Oligarchy" is also used as a collective term to denote all the individual members of the small corrupt ruling group in such a system. The term always has a negative or derogatory connotation in both contemporary and classical usage, in contrast to aristocracy (which sometimes has a derogatory connotation in modern usage, but never in classical).