Kind of short huh? Okay here goes.
1. Gettysburg was a turning point in the war because it was a big loss for Lee's army. Up until this point of the war, Lee's army hadn't really suffered any major kind of defeat. Sure there were losses here and there for the south, but nothing on the scale that Gettysburg was. From this point forward Lee and his army was on the defensive.
2. The Southern army not only lost the battle, but they lost a huge number of soldiers. The south's population numbers were not as high as the north, so it was tougher for the south to recover from massive soldier losses.
3. The Battle of Gettysburg killed 6 confederate generals. The union lost general as well, but again, the Confederate Army couldn't withstand and replace the losses as well as the Union. Plus it's always been considered that the Confederacy had more talented officers in the first place, so losing them was always difficult.
I don't disagree with anything in the first answer, but my understanding is that maybe the most important point about the battle is that it stopped the South's invasion of the North and put the CSA back on the defensive. This was important because the CSA might have been able to make the US settle for some sort of peace negotiations.
If the South had won at Gettysburg, its army could have headed for major population centers in the North and even to Washington, D.C. This might have pushed the North to accept peace overtures that the South was putting forward at the time. It might have even induced a European country or two to recognize the Confederacy. The fact that Gettysburg prevented these outcomes is a major reason why it is a turning point in this war.