I've just run across an interesting article in Foreign Affairs about an interesting, vital, yet simple question...

what exactly is terrorism?

It turns out that the answer is extremely tricky, even if one uses the commonly accepted definitions of "use of force by sub-state actors" or "use of force against civilians". And no matter how you define it, those who you definitely would want to classify as terrorists slip through the cracks, whereas others end up wearing the label that fit it uncomfortably at best.

It's a good read- I'd suggest checking it out.