Someone who uses violence, mayhem, and destruction — or the threat of those things — to coerce people or countries into taking a certain action is a terrorist. A terrorist may be motivated by religious fervor, politics, or just plain old-fashioned greed.
Terrorist has at its root the word "terror," which comes from the Latin word terrorem, meaning great fear. Great fear is exactly what terrorists hope to create so they can manipulate the situation to their benefit. The label terrorist is a subjective one. To the British, the American colonists who destroyed shiploads of tea in Boston Harbor in 1773 were terrorists. To colonists, they were patriots and heroes.