I like how alot of people here in the thread are immediately lumping terrorists into the single category of Muslim! Hahahah!
But anyway. The question of "what is terrorism" or "what is a terrorist" is actually a complicated one. You have to remember that in the mind of the terrorist, his cause is a just one. And if the cause is just, then how is terrorism much different from the War Of Independence in America?
Ironically, terrorism has worked in the past. Terrorism is a sort of way to say "you won't let us have our fair share? You won't let us sit at the table? Then we're going to blow you up until someone listens to us and we get a place at the table!".
It worked for the IRA.