I agree with filmfann, but would also like to add that the motive behind the act is often what characterises it. For example, the mafia is in it for the money, they would never harm anyone if there was no profit (at least an indirect one). They do use fear, but even then it is to ultimately make money by selling protection.
Terrorist attacks are politically motivated. The word “terrorist” is simply the term used by the victim. The perpetrators call themselves “freedom fighters” (as in the US-sponsored mujahedin in Afghanistan and Kosovo, before their interests took different paths in 2000). The “brave revolutionaries” who organised the Boston Tea Party were nothing more than “dirty terrorists” as far as His Majesty the Rightful Owner of the Colonies was concerned.
Whichever side you’re on, there is certainly a difference between brave/cowardly/dirty terrorists/guerillas/rebels and common criminals who are in it for the money.