"The unlawful use or threatened use of force or violence by a person or an organized group against people or property with the intention of intimidating or coercing societies or governments, often for ideological or political reasons " (Dictionary.com).
Terrorism can be many things. It can be an attack, a killing or capturing of the enemy, destruction of property, hijacking, or the threat of doing so. Sometimes it is an organization that extreme and dangerous as to take civilian (non-military) casualties.
What Terrorism is Not
A terrorist is not a guerrilla or person that takes up arm and then opposes an invading or occupying army on his or her own soil is not a terrorist. Instead, they are a resistance fighter or insurgent - thinking of freedom, thus freedom fighter. For instance, in the 1800s, the American Indians, or early settlers pacifying the Australian Aboriginals, despite using unconventional warfare against westward expansion could not be considered terrorists.
Of recent, the term is often overused with some claiming that acts like children bringing weapons to school or environmental extremists defacing SUVs count as "terrorism". (A separate term, terrist, is sometimes used to refer to acts against property in the defence of Earth.)
Despite the techinical definition, it is usually it is a government that defines what terrorism is in the law - which is not the same as doing so morally. The US State Department has a definition of terrorism and a list of terrorist organizations (mainly right-wing islamic groups and left-wing insurgencies).
Terrorism is widely seen as a dangerous Bad Thing, because it endangers physical security. The issue is complex, however it can never be disputed that terrorism is a human rights violation that exploits vulnerabilities.
Causes of Terrorism
Terrorism is motivated by desperation and then towards political goals, often with the hope to defeat or alter the policies of a government. Terrorists often feel that they have been shut out of the "legitimate" political processes in the past and feel that carrying out acts of violence and destruction are the only way they will be taken seriously enough to change public opinion. Terrorists may want to hijack the mass media so their agenda can be progressed. In this way their terrible act of violence become their public relations strategy at the same time.
Many people believe that the wasteful consumption of oil for unrestricted personal usage fuels Islamic terrorism. Without the overwhelming desire to control the Middle East for "national security" purposes, western nations wouldn't be secretly collaborating with or interfering in the affairs of oil rich arab nations.
Disputes over illegal land occupations and other oppression can escalate into terrorism. For example, the largest terorrist attack in history, according to it's chief planner, Osama Bin Laden, was inspired by the oppression and tyranny of the American-Israeli coalition against his people in Palestine and Lebanon.
Terrorism can be brought about as vengance for previous violent activities, similar to vigilantism. Some believe terrorism stems from a lack of education or from systemic poverty. xenophobia and zealotry can lead to Terrorism. Failures of the United Nations and a lack of enforcement and accountability can also lead to terrorism.
Governments and Terrorism
Sometimes Governments are accused of being Terrorists, as in the case of the United States in Iraq and Afghanistan, and Israel in Palestinian territory. In 1986, the US was convicted of terrorism (pdf) against Nicaragua by the World Court. The US had supported the contras with military training and equipment, who then attacked the left-wing Sandinista government in Nicaragua. Note that the US provides large "military assistance" to countries with left-wing insurgencies. The School_of_the_Americas is a training ground run by the US for anti-insurgency forces.
Success of Terrorism
It can be argued that Terrorism does not need to be successful to be advocated since often it is already based upon those of extremist view. However, some have suggested that the idea of a separate Palistinian state would never have come about without terrorism. Compared to the ethnic cleansing of the Armenians by the Turks between 1900 to 1923, who would also very much like their own separate state.
Unfortunately, many who practice terrorism often martyr those they harm and turn public opinion against themselves, acheiving the opposite of their aims. Whatever the outcome, Terrorism is almost always a step closer to chaos and fear rather than order and safety.