Terrorism is something that has plagued societies for ages. However, today its forms and means have both evolved to make it the most feared acts of modern times. Terrorism is defined by the US Department of Defence as:
"the unlawful use of -- or threatened use of -- force or violence against individuals or property to coerce or intimidate governments or societies, often to achieve political, religious, or ideological objectives." 
Such a form of terrorism is called State Terrorism.
It would not be wrong to say that 'terrorism' and 'terrorist' are both relative terms. Their meanings differ from people to people, society to society. Therefore it's not surprising that one man's terrorist is another man's freedom fighter!
'On one point, at least, everyone agrees: terrorism is a pejorative term. It is a word with intrinsically negative connotations that is generally applied to one's enemies and opponents, or to those with whom one disagrees and would otherwise prefer to ignore. 'What is called terrorism , thus seems to depend on one's point of view. Use of the term implies a moral judgment; and if one party can successfully attach the label terrorist to its opponent, then it has indirectly persuaded others to adopt its moral viewpoint.' Hence the decision to call someone or label some organization terrorist' becomes almost unavoidably subjective, depending largely on whether one sympathizes with or opposes the person/group/cause concerned. If one identifies with the victim of the violence, for example, then the act is terrorism. ...