To make a decision is to make up your mind about something. To act with decision is to proceed with determination, which might be a natural character trait.
Decision originally comes from the Latin decidere ("determine"). You make decisions every day: what to wear, what to eat, how to spend your money, who to vote for, what movie to go to. A court judge makes a decision in a trial (and actually "hands down" or "announces" that decision). If judges make the decision in a boxing match, the winner wins "by decision." More loosely, decision can also refer to the outcome of any game or contest.