I think the difference is the intent. If the intent of the act is the death (or other outcome) of a group of individuals then the act would be mass murder. If however the intent of the act is to use these acts to cause terror (the deaths are merely a means to that end) then the act is terrorism.

Terror has been a tool of a number of political groups over the years. An easy example would be when the IRA bombing campaign moved outside of London after the ring of steel was constructed. The destruction of property, or the deaths of individuals themselves in London weren't the aims, it was the threat/fear/terror that anyone could become a victim. The terror was a lever to force government response.