A terrorist is someone who employs violence in the fight for a belief, whether it be political, social or religious. The US government defines terrorism as the unlawful use or threatened use of force or violence to intimidate or coerce governments or societies often for ideological, religious or political reasons.
Terrorists are weaker in number and arms than the police or military force that they fight against, so they use subterfuge and target "soft" targets such as civilians sometimes causing terrible suffering and death among people who are not even involved in their fight. Their goal is to destabilize the ruling government and bring about a revolutionary change in the political system. Terrorists cannot survive without at least some help from the local population. As Che Geuvara suggested, the local population is the "sea" in which the terrorist "fish" swim. This evokes a good image for something that is vile.
Terrorists can operate when small numbers of supporters harbour them and the majority of the population despise them.
Terrorists and freedom fighters
Terrorism isn’t precisely defined. If one agrees with the goal of a terrorist organization, one deems them "freedom fighters" or "freedom lovers in support of regime change." In some wars both sides accuse the other side of terrorism.
Why terrorists attack western countries
Right wing Americans would have you believe that terrorists attack the certain Western countries because they hate our freedom. But as the brilliant comedic mind David Cross said, "If the terrorists hated freedom, then the Netherlands would be fucking dust." Here's another nugget of wisdom from David: "I don't think Osama bin Laden sent those planes to attack us because he hated our freedom. I think he did it because of our support for Israel, our ties with the Saudi family and our military bases in Saudi Arabia. You know why I think that? Because that's what he fucking said! Are we a nation of 6-year-olds? Answer: yes." In other words, Islamic terrorism has a lot to do with American foreign policy. This truth has been revealed not just by liberals like Cross, but by good freedom-loving conservatives like Ron Paul.
The known terrorists still out there need to be captured and killed as Osama Bin Laden was. We can't change our foreign policies to fit terrorist demands because that just gives in to the terrorists.