A "state" has a legitimate monopoly on the use of force. In his lecture “Politics as a Vocation" (1918), the German sociologist Max Weber defines the state as a “human community that (successfully) claims the monopoly of the legitimate use of physical force within a given territory."
In practice, entities recognized as states typically have fixed boundaries that normally exhibit only minor variations on a time scale of decades. States typically have populations of millions of people. Although states are not immortal, they typicallly persist for many decades, if not for several centuries. At the dawn of the Third Millenium, there were approximately 200 such states in existence.
The various militant outfits collected here as paramilitary groups are not states, though many of them would like to become states. They range in scope from small terrorist groups to rather large insurgencies. In contrast to states, they either control no territory or assert influence over a range of territory that changes with time, and typically has a population measured in thousands not millions. These militant outfits seldom persist for more than a few decades, as militants are killed faster than they are recruited, or as initial enthusiasm fades. And unlike the states which are their enemies, there does not seem to be any upper limit to the number of these outfits, a number which has kept growing in recent decades.
Terrorism should not be confused with insurrection; if it were the same, all those who fight oppression would be labeled as terrorists. One of the more damaging cliches making the rounds is that "One man's terrorist is another's Freedom Fighter." The principle targets of terrorists are innocent civilians or unarmed officials who are killed in order to terrorize populations or goad the government into unwarranted repression. Terrorism may be the weapon of the weak, a prelude to insurrection or insugency, but most militant outfits fail to graduate to these higher levels of struggle.