Putting aside all moral and legal justifications for the moment, from a purely practical standpoint, the purpose of military activity is to disable your opponent's ability to resist. You target his troops, weapons, logistics, communications, intelligence, command assets, supply, even the industrial capacity and civil infrastructure of his country, in order to cripple his ability to fight. Keep in mind, I am not addressing any questions of morality or ethics here. That is another issue altogether. It is simply a standard exposition of military doctrine.

Terrorism is fundamentally different. It seeks to intimidate civilians and non-combatants, particularly the innocent, to terrorize a population in order to change policy or public opinion. It deliberately targets those sectors of a society with no "military value", who have the least influence on government policy, knowing they are the most weakly defended. They do not kill or wound or otherwise disable an enemy so he can't fight, they attack his children and torture captives to induce a feeling of fear and helplessness in precisely those least able to endure it.

When I went to Cuba as a child to visit my relatives in 1958, the rebels blew up a bomb in a supermarket next door to the cinema where my aunt and I were watching "Davy Crockett and the River Pirates". Fortunately, it was a Sunday and the market was closed; the damage was severe but there were no injuries or deaths. I don't doubt this was due to the humanity of the rebels, or that particular cell at least, who did their best to avoid civilian casualties.

The theater and the market shared a wall, and the explosion sounded like it had occurred in the auditorium. There was a near-panic as the audience rushed to the exits to get out of the building. I asked my aunt later why they had acted so precipitously, since regardless of where the detonation occurred, the emergency was clearly over. She replied that the rebels often placed bombs in pairs, staggered to go off a few minutes apart, so that people rushing in to help the victims would be cut down in the second blast. And the audience was well aware of that. THAT is the difference between war and terrorism.

Terrorism is to war as torture is to combat. Yes, there is a difference, and you cannot ignore that difference simply because you don't like the politics of the victims, or because you are sympathetic to the cause of the terrorists.