as for my understanding, there's two kinds of terrorism,
first, to be clear, i don't consinder the palastinians or the iraqiq or the others who fight the invader in their countries terrorists, i consider them as in any other war, victims, chouhadae=means who fight to save his country/to learn and discover/ to help people/to do good things like fixing lands and cities and stuff. etc... in this case chahid is who dies when he's fighting the invaders
now, the first kind of terrorism, the terrorism with a reason: for example, in iraq, u.s.a invaded them, killed their families, their friends, destroyed their cities, etc... people, will get dessepred when all their family dies, so, they just say, what will i lose if i bombed my self and killed some army troops, at least i'll help to save my country, that how he'll think, cause he lost everything, and he have nothing left to loose, or, to fight to protect his family, and his people, as an example, imagine, a country invaded your country, your country have no army, and very weak, the invader is stealing lands, killing people, etc etc etc... what would you do, aren't you going to think about fighting? well, here, terrorism, is not the fighting in side the country, so, when those people get desepered and etc... and their hearts get felled with hate for the invader, as i explained, they will go to attack the invader's country in the heart, and he wan't consider any children or any women, cause he became desepered, also he'll say "hey, they killed my family, they killed my firends,they killed my people, they killed innocent, why shouldn't i do the same for them, let them taste how does it feal" personally, i dont consider this pure terrorism, (for islam war laws, you must never kill women, children or old people, should never destroy a building, destroy lands, or even destroy a tree, or kill an animal for no reason [but if it was a military base, it's not a problem to destroy it and it's troops)]
now about the real terrorsim, the first cause, is you, not you, i mean u.s.a, it started war, secong those groups, are not doing it to defend their country as a primary mission, but to try cause fear, revenge, also they have their own good for that, like trying to win money and steal it, just like mafia, or a criminal groups that try to get money and have other purpuces with illegal ways, but those terrorists are just a higher degree than mafias, they are more agressive, and do what they do by the name of relegion and (Allah/God)(who created every thing)