Seeking power & wealth is one of the reasons. Rigidity in Religions, Illiteracy and Poverty are the other main reasons behind it.
So far as today's Terrorism is concerned it definitely has a Religious nature... I think it's a common human as well as animal behavior to not tolerate "the different one", i.e. difference of looks, of physique, of nature, of beliefs etc. In human beings it comes forward as religion, Religion is a set of beliefs & principles to behave to which one gets adhered. When he faces someone who has different beliefs & behavior patterns, he gets worried, and it's a natural instinct of all animals.
And to prove the superiority of his beliefs & behavior patterns, to hide his own weaknesses and to avoid the superiority of the other one he wages a war against the second person and starts such cruel attacks, what terrorists are doing these days.This is one of the most important causes of War which humanity has seen from the start.
Another reason is Illiteracy !!. which causes an inability to think properly and in just manner. An illiterate person is like a sheep in a herd, can be directed towards any direction.
Third one is Poverty !! for money people can do anything, today most of the terrorist organizations are paying a lucrative amount of money to the trainees/terrorists, remember what Azmal Kasab has said.
Today majority of terrorists belong to Islamic background, and are worried about their future because the World is changing fast but due to the rigidity found in their beliefs they can't compete with the World. So they are fighting (waged Jihad) in the name of Islam to hide their own weaknesses.