Terrorism is about when someone actually harms other lives/property by exploding stuff, or by using weapons. But extremism is entirely different approach. Extremism starts when you follow some one or society beliefs, ideology , norms at peak level. you would have seen the word "mayana rawi" equality in many haidth books, so going beyond this mayana rawi is called extremism. All other society people call us extremism because they think we are following our faith above the required level. Similarly if someone follows west culture at extreme , this is also called extremism. So it is concluded that extremism starts when you go above the moderate level in any society.