We don't call them DUmmies for nothing.
Terrorist: Someone who uses violence for political purposes. They are making a statement about their political views and are trying to sway the population into surrender in a war of ideas. Take the Palestinian / Israeli conflict. The gunman in the school shooting was acting politically and was affiliated with an anti-Israeli militant group. That's why his actions were those of a terrorist.
Domestic Shooter: Someone who has no political motives and is not part of any particular political group trying to gain notoriety or media attention for their "cause". They are typically individuals who broke up with their girlfriend, feel someone has wronged them and are seeking revenge or are just plain mentally deranged.
Now chew on that for a while, DUmmies, and see if your peanut brains can put it all together.