Not all terrorist, but most contemporary terrorist are Islamic fundamentalist and use a distorted form of this religion to their needs.
Terrorism is a method of achieving political goals through acts of mass terror. Czar Russia terrorism, for instance, was aimed at the assassination of key officials and czars themselves. Also IRA etc.
But nowadays terrorism has become international or global as globalization has taken place. There are new ways of recruiting people. As we see they don't need to make bombs now. A vehicle and a knife are enough to kill many people. That's the scary part. A terrorist attack may occur anywhere in the world and nobody's safe.