Assuming that you are talking about Middle-Eastern terrorism (the kind of terrorism we saw on display on 9/11)... I would say that most terrorists fall into the label of "moderates" or "extremists". The latter being more common and more likely to commit acts of actual terror.
In a nutshell, the extremists want Israel completely destroyed as a nation, the permanent "elimination" of all Jews and the establishment of Muslim regimes to control no less than every single country in the world. This would coincide with the worldwide implementation of Sharia Law. Every single facet of everyone's lives would be controlled by the church, including all government and private business. In their eyes, this is what "Allah wants", and the ends justify the means, even if terrorism is contrary to what their own religion teaches. Needless to say, other religions would be banned by punishment of death.
On the other side of the fence are the "moderate" terrorists. They are not so hell-bent on world domination or the killing of all Jews, they mostly just want the state of Israel eliminated and the lands that they used to occupy returned to them. They want an immediate end of all aid to Israel by the United States. They want self-rule over their own lands and a permanent Muslim government established to rule over it. They want all non-muslim (infidels) and their armies expelled from the region.
The current president of Iran Mahmoud Ahmadinejad, definitely falls into the "extremist" category, which is why Iran is seen as such a huge threat. Eventually, one of these terrorist nations or groups is going to amass enough nukes or chemical/biological weapons to be able to achieve their goal, most likely. Thus, Israel (and the U.S.) spend an immense amount of time and money fighting terrorism, in an attempt to at least keep Israel on the face of the map.