Max Weber provided the simplest definition: “A government is an institution that holds a monopoly on the legitimate use of violence.” Another, fancier way he phrased it is “A state is a human community that (successfully) claims the monopoly of the legitimate use of physical force within a given territory.” I kind of like using Weber’s perspectives on government and bureaucracy.
One of the classic writers on government was Machiavelli.
I must at the beginning observe that some of the writers on politics distinguished three kinds of government, viz., the monarchical, the aristocratic, and the democratic; and maintain that the legislators of a people must choose from these three the one that seems most suitable. Other authors, wiser according to the opinion of many, count six kinds of government, three of which are very bad, and three good in themselves, but so liable to be corrupted that they become absolutely bad. The three good ones are those we just named, the three bad ones result from the degradation of the other three, and each of them resembles its corresponding original, so that the transition from the one to the other is very easy.
Thus monarchy becomes tyranny; aristocracy lapses into oligarchy; and the popular government lapses readily into licentiousness. So that a legislator who gives to a state which he founds, either of these three forms of government, constitutes it but for a brief time; for no precautions can prevent either one of the three that are reputed good, from degenerating into its opposite kind, so great are in those attractions and resemblances between the good and the evil.”
Niccolo Machiavelli, Discourses
There is more to Machiavelli than his simple quotations – but I’ll end with one: “Princes and governments are far more dangerous than other elements within society.” –Niccolo Machiavelli
Trego's Mountain Ear 