Mass media is absolutely needed. Without journalist you can’t have terrorist. This why terrorism is a thing from the modern world, and not before journalist was invented.
You need people to care about what you are doing. If you do this terror thing and nobody know or notice. you are a very bad terrorist. Real terrorism to be efective, need to be broadcasted on the news. So maybe you put a bomb in a dog, you want everybody in the city to know about it, everybody in the country, everybody in the world.
If you have a group of people that is happy with just causing damage, thats vandalism, not terrorism. Is the political message that turns simple vandalism into terrorist.
Also rarely you can confuse a guerrilla with terrorism, lol. A guerrilla have a static group of soldiers and assets, and use these assets to fight a asymetric war destroying assets from the enemy, personal if they can. They play a war thing where objectives are choosen for their militar value. A Terrorism can attack a empy building of offices that is choosen for the symbolic value, even if has zero militar value.
TL;DR:
How this group choosen their objectives?
random or no political message => vandalism group
political message => terrorism
militar value => guerrilla
on the real world there can be some mixing, a guerrilla can behave like a vandalism group once they have conquered a city, or maybe do some terrorism of their own. But still words have a meaning, ignore the idiots that try to confuse things for themselves and everybody else.
I am sorry if this sound insulting obvious, but a terrorism group is supposed to cause terror/fear. Somebody that make you just angry is not a terrorist. A terrorist is not somebody that make other people life slightly uncorfotable. Also a terrorism is a terrorist for their actions. Somebody that all he do is talk, he is not a terrorist (Can be something else, like a propaganda dude).
postdata:
I like Aleck definition a lot, is compact and better than anything I have wrote.