What is the "terrorist's" ultimate goal?

Groups like the Taliban, Al'qaeda, the insurgents in Somalia etc. What is their ultimate goal? I know alot of them want to spread their form of Islam, or destroy the US, but why? What do they do after that? Provided they have the resources, are they out to take over the world? Force everyone to follow their religion? So you take out america and all western civilizations...what then? Everyone just goes home then? Their actions seem tactically unsound, unless they have a wider purpose. So does anyone know? Have they ever said what they want to accomplish other than raising hell?