Approaching a topic like 'Terrorism' requires a very open mind and the willingness to understand issues from multiple perspectives. A famous quote which many people have taken credit for comes to mind:
'One man's terrorist is another man's freedom fighter' - Multiple people
Terrorism can be defined as 'The use of violence and intimidation in the pursuit of political or religious aims'. If you give this some thought you will probably realise that the number of groups with different political and religious aims is huge, meaning there will always be and has always been, to some extent, terrorism.
Some common causes of terrorism are poverty, desire for land, fundamentalist religious beliefs (extreme!) amongst other less significant things. Different groups of people have different problems with Government's for different reasons, and in some cases these problems are big enough for them to violently react - especially in cases where governments refuse to listen.
You will no doubt be thinking 'Can terrorism ever be justified?'. As usual, there are 2 sides to every argument. We must appreciate some of the genuine causes which terror groups represent whilst condemning actions where innocent civilians are killed. However, if killing civilians would save more innocent civilian lives on the other side, is this justifiable? Big questions, deep answers. Four common disadvantages to terrorism are:
1. The Obscene number of casualties (often)
2. Civilian casualties (not just military)
3. It spreads fear and intolerance of 'other' religions
4. Gives religion a bad name/label
Whilst analysing each of the groups above, to give your investigation some substance you should think about each using the following structure and consider similarities/differences between each of them!
Aims and motives
Methods they use
Structure of the group and it's leadership
How each effected government reacted
With these things in mind you should be able to approach the topic with a good background framework