When a serious event happens, many refer to it as a tragic event. The use of the word in such instances, like the shootings and murders in Tuscon, are incorrect in this regard. The shootings were not tragedy.
So what is a tragedy then?
When we think of a tragedy, we usually think of someone like Sophocles, or even Shakespeare. They wrote perhaps the greatest tragedies in history. Aristotle writes in his Poetics that the tragedy is about cause and effect in some way. It is a moral commentary of what is possible, and how seemingly unconnected events may end up causing great calamity. There is a nice site at this university that explains Aristotle this way:
Tragedy depicts the downfall of a noble hero or heroine, usually through some combination of hubris, fate, and/[or] the will of the gods. The tragic hero’s powerful wish to achieve some goal inevitably encounters limits, usually those of human frailty (flaws in reason, hubris, society), the gods (through oracles, prophets, fate), or nature. Aristotle says that the tragic hero should have a flaw and/or make some mistake (hamartia). The hero need not die at the end, but he / she must undergo a change in fortune. In addition, the tragic hero may achieve some revelation or recognition (anagnorisis–“knowing again” or “knowing back” or “knowing throughout” ) about human fate, destiny, and the will of the gods. Aristotle quite nicely terms this sort of recognition “a change from ignorance to awareness of a bond of love or hate.”
So the thing about tragedy is that the hero (or really the person who is the main character) has some human flaw. The flaw is one that all humans could have in their character. The plays are such that they engross the audience because the audience is thinking “this misfortune could happen to me.” It could be that the main character is making choices that all seem correct, but end up making for much trouble for that character–thus the “knowing back” portion of the tragedy has a “lesson” element to it.