So here's the basic story. The israelites are all living peacefully when the Palestinians attack, and a big war ensues. david was a young adolescent at the time, somewhere between 12-16. His brothers all went off to war, and he stayed home to tend the family sheep.
the brothers go to the front line, where king solomon has worked out a deal with the Palestinians: a one-on-one match, between the best warriors from either country. The winner wins the entire war, meaning the retreat of the Palestinians or the conquering of Israel. Unfortunately, the Palestinians have a really, really big guy named Goliath (that was really his name, where the term comes from) who stood about 8-12 feet tall (it was in cubits, and no one really agrees on how much that really is). No one wanted to fight him. While everyone hides, Goliath makes all sorts of derogatory comments about the Israelites and their religion.
Meanwhile.....
David's father sends him to the front line with supplies for his brothers and the army. When he arrives, he finds the situation quite disturbing, and goes and tells king solomon that he will be the one to fight Goliath. He is a faithful boy, and believes God will protect him. He eventually convinces the king, and prepares himself by gathering seven smooth stones from a riverbed, along with a sling. When he faces Goliath, the giant is suitably humored. However, David slings a stone, and strikes Goliath between the eyes, killing him instantly. The Palestinians all flee in fear of David, who is a hero, and moves on to become the king david who makes up much more of the Old Testament.
The End